Added gofmt to format command

Fixes #716 #715

.dockerignore

@@ -1 +1 @@
-bin
+bin/*

.github/FUNDING.yml

@@ -0,0 +1 @@
+github: mikefarah

.github/workflows/release.yml

@@ -3,7 +3,6 @@ on:
   push:
     tags:
       - 'v*'
-      - 'dockerfix'
 
 jobs:
   publishGitRelease:
@@ -41,17 +40,7 @@ jobs:
       IMAGE_NAME: mikefarah/yq
     runs-on: ubuntu-latest
     steps:
-      - name: Get latest release tag      
-        uses: oprypin/find-latest-tag@v1
-        with:
-          repository: mikefarah/yq  # The repository to scan.
-          releases-only: true  # We know that all relevant tags have a GitHub release for them.
-        id: yq  
-        
-      - name: Clone source code
-        uses: actions/checkout@v2
-        with:
-          ref: ${{ steps.yq.outputs.tag }}
+      - uses: actions/checkout@v2
       
       - name: Set up QEMU
         uses: docker/setup-qemu-action@v1

Dockerfile.dev

@@ -6,26 +6,5 @@ RUN set -e -x \
     && /opt/devtools.sh
 ENV PATH=/go/bin:$PATH
 
-# install mkdocs
-RUN set -ex \
-    && buildDeps=' \
-        build-essential \
-        python3-dev \
-    ' \
-    && apt-get update && apt-get install -y --no-install-recommends \
-        $buildDeps \
-        python3 \
-        python3-setuptools \
-        python3-wheel \
-        python3-pip \
-    && pip3 install --upgrade \
-        pip \
-        'Markdown>=2.6.9' \
-        'mkdocs>=0.16.3' \
-        'mkdocs-material>=1.10.1' \
-        'markdown-include>=0.5.1' \
-    && apt-get purge -y --auto-remove $buildDeps \
-    && rm -rf /var/lib/apt/lists/*
-
 ENV CGO_ENABLED 0
 ENV GOPATH /go:/yq

Makefile

@@ -17,6 +17,7 @@ help:
 	@echo '    make vendor          Install dependencies to vendor directory.'
 	@echo '    make format          Run code formatter.'
 	@echo '    make check           Run static code analysis (lint).'
+	@echo '    make secure          Run gosec.'
 	@echo '    make test            Run tests on project.'
 	@echo '    make cover           Run tests and capture code coverage metrics on project.'
 	@echo '    make clean           Clean the directory tree of produced artifacts.'
@@ -84,6 +85,10 @@ format: vendor
 check: format
 	${DOCKRUN} bash ./scripts/check.sh
 
+.PHONY: secure
+secure: 
+	${DOCKRUN} bash ./scripts/secure.sh
+
 .PHONY: test
 test: check
 	${DOCKRUN} bash ./scripts/test.sh
@@ -96,11 +101,6 @@ cover: check
 	@find cover -type d -exec chmod 755 {} \; || :
 	@find cover -type f -exec chmod 644 {} \; || :
 
-.PHONY: build-docs
-build-docs: prepare mkdocs.yml mkdocs/*
-	${DOCKRUN} mkdocs build
-	@find docs -type d -exec chmod 755 {} \; || :
-	@find docs -type f -exec chmod 644 {} \; || :
 
 .PHONY: release
 release: xcompile

README.md

@@ -103,7 +103,7 @@ yq() {
 
 ### Go Get:
 ```
-GO111MODULE=on go get github.com/mikefarah/yq
+GO111MODULE=on go get github.com/mikefarah/yq/v4
 ```
 
 ## Community Supported Installation methods
@@ -119,6 +119,8 @@ See [webi](https://webinstall.dev/)
 Supported by @adithyasunil26 (https://github.com/webinstall/webi-installers/tree/master/yq)
 
 ### Windows:
+[![Chocolatey](https://img.shields.io/chocolatey/v/yq.svg)](https://chocolatey.org/packages/yq)
+[![Chocolatey](https://img.shields.io/chocolatey/dt/yq.svg)](https://chocolatey.org/packages/yq)
 ```
 choco install yq
 ```
@@ -151,19 +153,21 @@ sudo apt install yq -y
 Supported by @rmescandon (https://launchpad.net/~rmescandon/+archive/ubuntu/yq)
 
 ## Features
+- [Detailed documentation with many examples](https://mikefarah.gitbook.io/yq/)
 - Written in portable go, so you can download a lovely dependency free binary
 - Uses similar syntax as `jq` but works with YAML and JSON files
 - Fully supports multi document yaml files
 - Colorized yaml output
-- [Deeply traverse yaml](https://mikefarah.gitbook.io/yq/v/v4.x/traverse)
-- [Sort yaml by keys](https://mikefarah.gitbook.io/yq/v/v4.x/sort-keys)
-- Manipulate yaml [comments](https://mikefarah.gitbook.io/yq/comment-operators), [styling](https://mikefarah.gitbook.io/yq/style), [tags](https://mikefarah.gitbook.io/yq/tag) and [anchors and aliases](https://mikefarah.gitbook.io/yq/anchor-and-alias-operators).
+- [Deeply traverse yaml](https://mikefarah.gitbook.io/yq/operators/traverse-read)
+- [Sort yaml by keys](https://mikefarah.gitbook.io/yq/operators/sort-keys)
+- Manipulate yaml [comments](https://mikefarah.gitbook.io/yq/operators/comment-operators), [styling](https://mikefarah.gitbook.io/yq/operators/style), [tags](https://mikefarah.gitbook.io/yq/operators/tag) and [anchors and aliases](https://mikefarah.gitbook.io/yq/operators/anchor-and-alias-operators).
 - [Update yaml inplace](https://mikefarah.gitbook.io/yq/v/v4.x/commands/evaluate#flags)
-- [Complex expressions to select and update](https://mikefarah.gitbook.io/yq/v/v4.x/select#select-and-update-matching-values-in-map)
+- [Complex expressions to select and update](https://mikefarah.gitbook.io/yq/operators/select#select-and-update-matching-values-in-map)
 - Keeps yaml formatting and comments when updating (though there are issues with whitespace)
 - [Convert to/from json to yaml](https://mikefarah.gitbook.io/yq/v/v4.x/usage/convert)
 - [Pipe data in by using '-'](https://mikefarah.gitbook.io/yq/v/v4.x/commands/evaluate)
 - [General shell completion scripts (bash/zsh/fish/powershell)](https://mikefarah.gitbook.io/yq/v/v4.x/commands/shell-completion)
+- [Reduce](https://mikefarah.gitbook.io/yq/operators/reduce) to merge multiple files or sum an array or other fancy things.
 
 ## [Usage](https://mikefarah.gitbook.io/yq/)
 

cmd/evaluate_all_command.go

@@ -17,6 +17,9 @@ func createEvaluateAllCommand() *cobra.Command {
 		Example: `
 # merges f2.yml into f1.yml (inplace)
 yq eval-all --inplace 'select(fileIndex == 0) * select(fileIndex == 1)' f1.yml f2.yml		
+
+# use '-' as a filename to read from STDIN
+cat file2.yml | yq ea '.a.b' file1.yml - file3.yml
 `,
 		Long: "Evaluate All:\nUseful when you need to run an expression across several yaml documents or files. Consumes more memory than eval",
 		RunE: evaluateAll,
@@ -42,14 +45,21 @@ func evaluateAll(cmd *cobra.Command, args []string) error {
 		colorsEnabled = true
 	}
 
-	if writeInplace && len(args) < 2 {
+	firstFileIndex := -1
+	if !nullInput && len(args) == 1 {
+		firstFileIndex = 0
+	} else if len(args) > 1 {
+		firstFileIndex = 1
+	}
+
+	if writeInplace && (firstFileIndex == -1) {
 		return fmt.Errorf("Write inplace flag only applicable when giving an expression and at least one file")
 	}
 
 	if writeInplace {
 		// only use colors if its forced
 		colorsEnabled = forceColor
-		writeInPlaceHandler := yqlib.NewWriteInPlaceHandler(args[1])
+		writeInPlaceHandler := yqlib.NewWriteInPlaceHandler(args[firstFileIndex])
 		out, err = writeInPlaceHandler.CreateTempFile()
 		if err != nil {
 			return err

cmd/evalute_sequence_command.go

@@ -21,6 +21,9 @@ yq e '.a.b | length' f1.yml f2.yml
 # prints out the file
 yq e sample.yaml 
 
+# use '-' as a filename to read from STDIN
+cat file2.yml | yq e '.a.b' file1.yml - file3.yml
+
 # prints a new yaml document
 yq e -n '.a.b.c = "cat"' 
 
@@ -62,14 +65,21 @@ func evaluateSequence(cmd *cobra.Command, args []string) error {
 		colorsEnabled = true
 	}
 
-	if writeInplace && len(args) < 2 {
+	firstFileIndex := -1
+	if !nullInput && len(args) == 1 {
+		firstFileIndex = 0
+	} else if len(args) > 1 {
+		firstFileIndex = 1
+	}
+
+	if writeInplace && (firstFileIndex == -1) {
 		return fmt.Errorf("Write inplace flag only applicable when giving an expression and at least one file")
 	}
 
 	if writeInplace {
 		// only use colors if its forced
 		colorsEnabled = forceColor
-		writeInPlaceHandler := yqlib.NewWriteInPlaceHandler(args[1])
+		writeInPlaceHandler := yqlib.NewWriteInPlaceHandler(args[firstFileIndex])
 		out, err = writeInPlaceHandler.CreateTempFile()
 		if err != nil {
 			return err
@@ -102,7 +112,7 @@ func evaluateSequence(cmd *cobra.Command, args []string) error {
 			err = streamEvaluator.EvaluateFiles(processExpression(""), []string{args[0]}, printer)
 		}
 	default:
-		err = streamEvaluator.EvaluateFiles(args[0], args[1:], printer)
+		err = streamEvaluator.EvaluateFiles(processExpression(args[0]), args[1:], printer)
 	}
 	completedSuccessfully = err == nil
 

cmd/version.go

@@ -11,7 +11,7 @@ var (
 	GitDescribe string
 
 	// Version is main version number that is being run at the moment.
-	Version = "4.4.1"
+	Version = "4.9.3"
 
 	// VersionPrerelease is a pre-release marker for the version. If this is "" (empty string)
 	// then it means that it is a final release. Otherwise, this is a pre-release

examples/data1.yaml

@@ -1,4 +1,6 @@
-a: simple # just the best
-b: [1, 2]
-c:
-  test: 1
+a:
+  key1: "value1"
+  key2: 2.6
+  ab:
+    key1: 6
+    key2: "h"

github-action/Dockerfile

@@ -1,4 +1,4 @@
-FROM mikefarah/yq:4.4.1
+FROM mikefarah/yq:4.9.3
 
 COPY entrypoint.sh /entrypoint.sh
 

go.mod

@@ -1,14 +1,14 @@
 module github.com/mikefarah/yq/v4
 
 require (
-	github.com/elliotchance/orderedmap v1.3.0
+	github.com/elliotchance/orderedmap v1.4.0
 	github.com/fatih/color v1.10.0
-	github.com/goccy/go-yaml v1.8.4
-	github.com/jinzhu/copier v0.1.0
-	github.com/spf13/cobra v1.1.1
+	github.com/goccy/go-yaml v1.8.9
+	github.com/jinzhu/copier v0.2.8
+	github.com/spf13/cobra v1.1.3
 	github.com/timtadh/data-structures v0.5.3 // indirect
 	github.com/timtadh/lexmachine v0.2.2
-	golang.org/x/sys v0.0.0-20210105210732-16f7687f5001 // indirect
+	golang.org/x/sys v0.0.0-20210317225723-c4fcb01b228e // indirect
 	gopkg.in/op/go-logging.v1 v1.0.0-20160211212156-b2cb9fa56473
 	gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b
 )

go.sum

@@ -36,8 +36,8 @@ github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/dgrijalva/jwt-go v3.2.0+incompatible/go.mod h1:E3ru+11k8xSBh+hMPgOLZmtrrCbhqsmaPHjLKYnJCaQ=
 github.com/dgryski/go-sip13 v0.0.0-20181026042036-e10d5fee7954/go.mod h1:vAd38F8PWV+bWy6jNmig1y/TA+kYO4g3RSRF0IAv0no=
-github.com/elliotchance/orderedmap v1.3.0 h1:k6m77/d0zCXTjsk12nX40TkEBkSICq8T4s6R6bpCqU0=
-github.com/elliotchance/orderedmap v1.3.0/go.mod h1:8hdSl6jmveQw8ScByd3AaNHNk51RhbTazdqtTty+NFw=
+github.com/elliotchance/orderedmap v1.4.0 h1:wZtfeEONCbx6in1CZyE6bELEt/vFayMvsxqI5SgsR+A=
+github.com/elliotchance/orderedmap v1.4.0/go.mod h1:wsDwEaX5jEoyhbs7x93zk2H/qv0zwuhg4inXhDkYqys=
 github.com/fatih/color v1.7.0/go.mod h1:Zm6kSWBoL9eyXnKyktHP6abPY2pDugNf5KwzbycvMj4=
 github.com/fatih/color v1.10.0 h1:s36xzo75JdqLaaWoiEHk767eHiwo0598uUxyfiPkDsg=
 github.com/fatih/color v1.10.0/go.mod h1:ELkj/draVOlAH/xkhN6mQ50Qd0MPOk5AAr3maGEBuJM=
@@ -52,8 +52,8 @@ github.com/go-playground/locales v0.13.0/go.mod h1:taPMhCMXrRLJO55olJkUXHZBHCxTM
 github.com/go-playground/universal-translator v0.17.0/go.mod h1:UkSxE5sNxxRwHyU+Scu5vgOQjsIJAF8j9muTVoKLVtA=
 github.com/go-playground/validator/v10 v10.4.1/go.mod h1:nlOn6nFhuKACm19sB/8EGNn9GlaMV7XkbRSipzJ0Ii4=
 github.com/go-stack/stack v1.8.0/go.mod h1:v0f6uXyyMGvRgIKkXu+yp6POWl0qKG85gN/melR3HDY=
-github.com/goccy/go-yaml v1.8.4 h1:AOEdR7aQgbgwHznGe3BLkDQVujxCPUpHOZZcQcp8Y3M=
-github.com/goccy/go-yaml v1.8.4/go.mod h1:U/jl18uSupI5rdI2jmuCswEA2htH9eXfferR3KfscvA=
+github.com/goccy/go-yaml v1.8.9 h1:4AEXg2qx+/w29jXnXpMY6mTckmYu1TMoHteKuMf0HFg=
+github.com/goccy/go-yaml v1.8.9/go.mod h1:U/jl18uSupI5rdI2jmuCswEA2htH9eXfferR3KfscvA=
 github.com/gogo/protobuf v1.1.1/go.mod h1:r8qH/GZQm5c6nD/R0oafs1akxWv10x8SbQlK7atdtwQ=
 github.com/gogo/protobuf v1.2.1/go.mod h1:hp+jE20tsWTFYpLwKvXlhS1hjn+gTNwPg2I6zVXpSg4=
 github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b/go.mod h1:SBH7ygxi8pfUlaOkMMuAQtPIUF8ecWP5IEl/CR7VP2Q=
@@ -101,8 +101,8 @@ github.com/hashicorp/memberlist v0.1.3/go.mod h1:ajVTdAv/9Im8oMAAj5G31PhhMCZJV2p
 github.com/hashicorp/serf v0.8.2/go.mod h1:6hOLApaqBFA1NXqRQAsxw9QxuDEvNxSQRwA/JwenrHc=
 github.com/inconshreveable/mousetrap v1.0.0 h1:Z8tu5sraLXCXIcARxBp/8cbvlwVa7Z1NHg9XEKhtSvM=
 github.com/inconshreveable/mousetrap v1.0.0/go.mod h1:PxqpIevigyE2G7u3NXJIT2ANytuPF1OarO4DADm73n8=
-github.com/jinzhu/copier v0.1.0 h1:Vh8xALtH3rrKGB/XIRe5d0yCTHPZFauWPLvdpDAbi88=
-github.com/jinzhu/copier v0.1.0/go.mod h1:24xnZezI2Yqac9J61UC6/dG/k76ttpq0DdJI3QmUvro=
+github.com/jinzhu/copier v0.2.8 h1:N8MbL5niMwE3P4dOwurJixz5rMkKfujmMRFmAanSzWE=
+github.com/jinzhu/copier v0.2.8/go.mod h1:24xnZezI2Yqac9J61UC6/dG/k76ttpq0DdJI3QmUvro=
 github.com/jonboulle/clockwork v0.1.0/go.mod h1:Ii8DK3G1RaLaWxj9trq07+26W01tbo22gdxWY5EU2bo=
 github.com/json-iterator/go v1.1.6/go.mod h1:+SdeFBvtyEkXs7REEP0seUULqWtbJapLOCVDaaPEHmU=
 github.com/jstemmer/go-junit-report v0.0.0-20190106144839-af01ea7f8024/go.mod h1:6v2b51hI/fHJwM22ozAgKL4VKDeJcHhJFhtBdhmNjmU=
@@ -168,10 +168,9 @@ github.com/soheilhy/cmux v0.1.4/go.mod h1:IM3LyeVVIOuxMH7sFAkER9+bJ4dT7Ms6E4xg4k
 github.com/spaolacci/murmur3 v0.0.0-20180118202830-f09979ecbc72/go.mod h1:JwIasOWyU6f++ZhiEuf87xNszmSA2myDM2Kzu9HwQUA=
 github.com/spf13/afero v1.1.2/go.mod h1:j4pytiNVoe2o6bmDsKpLACNPDBIoEAkihy7loJ1B0CQ=
 github.com/spf13/cast v1.3.0/go.mod h1:Qx5cxh0v+4UWYiBimWS+eyWzqEqokIECu5etghLkUJE=
-github.com/spf13/cobra v1.1.1 h1:KfztREH0tPxJJ+geloSLaAkaPkr4ki2Er5quFV1TDo4=
-github.com/spf13/cobra v1.1.1/go.mod h1:WnodtKOvamDL/PwE2M4iKs8aMDBZ5Q5klgD3qfVJQMI=
+github.com/spf13/cobra v1.1.3 h1:xghbfqPkxzxP3C/f3n5DdpAbdKLj4ZE4BWQI362l53M=
+github.com/spf13/cobra v1.1.3/go.mod h1:pGADOWyqRD/YMrPZigI/zbliZ2wVD/23d+is3pSWzOo=
 github.com/spf13/jwalterweatherman v1.0.0/go.mod h1:cQK4TGJAtQXfYWX+Ddv3mKDzgVb68N+wFjFa4jdeBTo=
-github.com/spf13/pflag v1.0.3 h1:zPAT6CGy6wXeQ7NtTnaTerfKOsV6V6F8agHXFiazDkg=
 github.com/spf13/pflag v1.0.3/go.mod h1:DYY7MBk1bdzusC3SYhjObp+wFpr4gzcvqqNjLnInEg4=
 github.com/spf13/pflag v1.0.5 h1:iy+VFUOCP1a+8yFto/drg2CJ5u0yRoB7fZw3DKv/JXA=
 github.com/spf13/pflag v1.0.5/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
@@ -180,8 +179,9 @@ github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+
 github.com/stretchr/objx v0.1.1/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.3.0/go.mod h1:M5WIy9Dh21IEIfnGCwXGc5bZfKNJtfHm1UVUgZn+9EI=
-github.com/stretchr/testify v1.4.0 h1:2E4SXV/wtOkTonXsotYi4li6zVWxYlZuYNCXe9XRJyk=
 github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
+github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
+github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/subosito/gotenv v1.2.0/go.mod h1:N0PQaV/YGNqwC0u51sEeR/aUtSLEXKX9iv69rRypqCw=
 github.com/timtadh/data-structures v0.5.3 h1:F2tEjoG9qWIyUjbvXVgJqEOGJPMIiYn7U5W5mE+i/vQ=
 github.com/timtadh/data-structures v0.5.3/go.mod h1:9R4XODhJ8JdWFEI8P/HJKqxuJctfBQw6fDibMQny2oU=
@@ -255,10 +255,9 @@ golang.org/x/sys v0.0.0-20190507160741-ecd444e8653b/go.mod h1:h1NjWce9XRLGQEsW7w
 golang.org/x/sys v0.0.0-20190606165138-5da285871e9c/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20190624142023-c5567b49c5d0/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20200116001909-b77594299b42/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20200223170610-d5e6a3e2c0ae h1:/WDfKMnPU+m5M4xB+6x4kaepxRw6jWvR5iDRdvjHgy8=
 golang.org/x/sys v0.0.0-20200223170610-d5e6a3e2c0ae/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.0.0-20210105210732-16f7687f5001 h1:/dSxr6gT0FNI1MO5WLJo8mTmItROeOKTkDn+7OwWBos=
-golang.org/x/sys v0.0.0-20210105210732-16f7687f5001/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210317225723-c4fcb01b228e h1:XNp2Flc/1eWQGk5BLzqTAN7fQIwIbfyVTuVxXxZh73M=
+golang.org/x/sys v0.0.0-20210317225723-c4fcb01b228e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.1-0.20180807135948-17ff2d5776d2/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
@@ -268,7 +267,6 @@ golang.org/x/tools v0.0.0-20180221164845-07fd8470d635/go.mod h1:n7NCudcB/nEzxVGm
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20190114222345-bf090417da8b/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20190226205152-f727befe758c/go.mod h1:9Yl7xja0Znq3iFh3HoIrodX9oNMXvdceNzlUR8zjMvY=
-golang.org/x/tools v0.0.0-20190311212946-11955173bddd h1:/e+gpKk9r3dJobndpTytxS2gOy6m5uvpg+ISQoEcusQ=
 golang.org/x/tools v0.0.0-20190311212946-11955173bddd/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
 golang.org/x/tools v0.0.0-20190312151545-0bb0c0a6e846/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
 golang.org/x/tools v0.0.0-20190312170243-e65039ee4138/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=
@@ -281,7 +279,6 @@ golang.org/x/tools v0.0.0-20190628153133-6cdbf07be9d0/go.mod h1:/rFqwRUd4F7ZHNgw
 golang.org/x/tools v0.0.0-20190816200558-6889da9d5479/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/tools v0.0.0-20190911174233-4f2ddba30aff/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/tools v0.0.0-20191012152004-8de300cfc20a/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
-golang.org/x/tools v0.0.0-20191112195655-aa38f8e97acc h1:NCy3Ohtk6Iny5V/reW2Ktypo4zIpWBdRJ1uFMjBxdg8=
 golang.org/x/tools v0.0.0-20191112195655-aa38f8e97acc/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
 golang.org/x/xerrors v0.0.0-20190717185122-a985d3407aa7/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 h1:go1bK/D/BFZV2I8cIQd1NKEZ+0owSTG1fDTci4IqFcE=
@@ -308,7 +305,6 @@ google.golang.org/grpc v1.19.0/go.mod h1:mqu4LbDTu4XGKhr4mRzUsmM4RtVoemTSY81AxZi
 google.golang.org/grpc v1.20.1/go.mod h1:10oTOabMzJvdu6/UiuZezV6QK5dSlG84ov/aaiqXj38=
 google.golang.org/grpc v1.21.1/go.mod h1:oYelfM1adQP15Ek0mdvEgi9Df8B9CZIaU1084ijfRaM=
 gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLkstjWtayDeSgw=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 h1:qIbj1fsPNlZgppZ+VLlY7N33q108Sa+fhmuc+sWQYwY=
 gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
@@ -319,11 +315,10 @@ gopkg.in/op/go-logging.v1 v1.0.0-20160211212156-b2cb9fa56473/go.mod h1:N1eN2tsCx
 gopkg.in/resty.v1 v1.12.0/go.mod h1:mDo4pnntr5jdWRML875a/NmxYqAlA73dVijT2AXvQQo=
 gopkg.in/yaml.v2 v2.0.0-20170812160011-eb3733d160e7/go.mod h1:JAlM8MvJe8wmxCU4Bli9HhUf9+ttbYbLASfIpnQbh74=
 gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.2 h1:ZCJp+EgiOT7lHqUV2J862kp8Qj64Jo6az82+3Td9dZw=
 gopkg.in/yaml.v2 v2.2.2/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.2.4/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
-gopkg.in/yaml.v2 v2.2.8 h1:obN1ZagJSUGI0Ek/LBmuj4SNLPfIny3KsKFopxRdj10=
-gopkg.in/yaml.v2 v2.2.8/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
+gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b h1:h8qDotaEPuJATrMmW04NCwg7v22aHH28wwpauUhK9Oo=
 gopkg.in/yaml.v3 v3.0.0-20210107192922-496545a6307b/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=

pkg/yqlib/all_at_once_evaluator.go

@@ -39,7 +39,11 @@ func (e *allAtOnceEvaluator) EvaluateCandidateNodes(expression string, inputCand
 	if err != nil {
 		return nil, err
 	}
-	return e.treeNavigator.GetMatchingNodes(inputCandidates, node)
+	context, err := e.treeNavigator.GetMatchingNodes(Context{MatchingNodes: inputCandidates}, node)
+	if err != nil {
+		return nil, err
+	}
+	return context.MatchingNodes, nil
 }
 
 func (e *allAtOnceEvaluator) EvaluateFiles(expression string, filenames []string, printer Printer) error {

pkg/yqlib/candidate_node.go

@@ -2,32 +2,37 @@ package yqlib
 
 import (
 	"fmt"
-	"strconv"
-	"strings"
 
 	"github.com/jinzhu/copier"
 	yaml "gopkg.in/yaml.v3"
 )
 
 type CandidateNode struct {
-	Node      *yaml.Node    // the actual node
-	Path      []interface{} /// the path we took to get to this node
-	Document  uint          // the document index of this node
+	Node      *yaml.Node     // the actual node
+	Parent    *CandidateNode // parent node
+	Path      []interface{}  /// the path we took to get to this node
+	Document  uint           // the document index of this node
 	Filename  string
 	FileIndex int
 	// when performing op against all nodes given, this will treat all the nodes as one
 	// (e.g. top level cross document merge). This property does not propegate to child nodes.
 	EvaluateTogether bool
+	IsMapKey         bool
 }
 
 func (n *CandidateNode) GetKey() string {
-	return fmt.Sprintf("%v - %v", n.Document, n.Path)
+	keyPrefix := ""
+	if n.IsMapKey {
+		keyPrefix = "key-"
+	}
+	return fmt.Sprintf("%v%v - %v", keyPrefix, n.Document, n.Path)
 }
 
 func (n *CandidateNode) CreateChild(path interface{}, node *yaml.Node) *CandidateNode {
 	return &CandidateNode{
 		Node:      node,
 		Path:      n.createChildPath(path),
+		Parent:    n,
 		Document:  n.Document,
 		Filename:  n.Filename,
 		FileIndex: n.FileIndex,
@@ -63,11 +68,10 @@ func (n *CandidateNode) UpdateFrom(other *CandidateNode) {
 	n.UpdateAttributesFrom(other)
 	n.Node.Content = other.Node.Content
 	n.Node.Value = other.Node.Value
-	n.Node.Alias = other.Node.Alias
-	n.Node.Anchor = other.Node.Anchor
 }
 
 func (n *CandidateNode) UpdateAttributesFrom(other *CandidateNode) {
+	log.Debug("UpdateAttributesFrom: n: %v other: %v", n.GetKey(), other.GetKey())
 	if n.Node.Kind != other.Node.Kind {
 		// clear out the contents when switching to a different type
 		// e.g. map to array
@@ -76,56 +80,22 @@ func (n *CandidateNode) UpdateAttributesFrom(other *CandidateNode) {
 	}
 	n.Node.Kind = other.Node.Kind
 	n.Node.Tag = other.Node.Tag
+	n.Node.Alias = other.Node.Alias
+	n.Node.Anchor = other.Node.Anchor
 
 	// merge will pickup the style of the new thing
 	// when autocreating nodes
 	if n.Node.Style == 0 {
 		n.Node.Style = other.Node.Style
 	}
-	n.Node.FootComment = n.Node.FootComment + other.Node.FootComment
-	n.Node.HeadComment = n.Node.HeadComment + other.Node.HeadComment
-	n.Node.LineComment = n.Node.LineComment + other.Node.LineComment
-}
 
-func (n *CandidateNode) PathStackToString() string {
-	return mergePathStackToString(n.Path)
-}
-
-func mergePathStackToString(pathStack []interface{}) string {
-	var sb strings.Builder
-	for index, path := range pathStack {
-		switch path.(type) {
-		case int, int64:
-			// if arrayMergeStrategy == AppendArrayMergeStrategy {
-			// sb.WriteString("[+]")
-			// } else {
-			sb.WriteString(fmt.Sprintf("[%v]", path))
-			// }
-
-		default:
-			s := fmt.Sprintf("%v", path)
-			var _, errParsingInt = strconv.ParseInt(s, 10, 64) // nolint
-
-			hasSpecial := strings.Contains(s, ".") || strings.Contains(s, "[") || strings.Contains(s, "]") || strings.Contains(s, "\"")
-			hasDoubleQuotes := strings.Contains(s, "\"")
-			wrappingCharacterStart := "\""
-			wrappingCharacterEnd := "\""
-			if hasDoubleQuotes {
-				wrappingCharacterStart = "("
-				wrappingCharacterEnd = ")"
-			}
-			if hasSpecial || errParsingInt == nil {
-				sb.WriteString(wrappingCharacterStart)
-			}
-			sb.WriteString(s)
-			if hasSpecial || errParsingInt == nil {
-				sb.WriteString(wrappingCharacterEnd)
-			}
-		}
-
-		if index < len(pathStack)-1 {
-			sb.WriteString(".")
-		}
+	if other.Node.FootComment != "" {
+		n.Node.FootComment = other.Node.FootComment
+	}
+	if other.Node.HeadComment != "" {
+		n.Node.HeadComment = other.Node.HeadComment
+	}
+	if other.Node.LineComment != "" {
+		n.Node.LineComment = other.Node.LineComment
 	}
-	return sb.String()
 }

pkg/yqlib/context.go

@@ -0,0 +1,68 @@
+package yqlib
+
+import (
+	"container/list"
+
+	"github.com/jinzhu/copier"
+)
+
+type Context struct {
+	MatchingNodes  *list.List
+	Variables      map[string]*list.List
+	DontAutoCreate bool
+}
+
+func (n *Context) SingleReadonlyChildContext(candidate *CandidateNode) Context {
+	list := list.New()
+	list.PushBack(candidate)
+	newContext := n.ChildContext(list)
+	newContext.DontAutoCreate = true
+	return newContext
+}
+
+func (n *Context) SingleChildContext(candidate *CandidateNode) Context {
+	list := list.New()
+	list.PushBack(candidate)
+	return n.ChildContext(list)
+}
+
+func (n *Context) GetVariable(name string) *list.List {
+	if n.Variables == nil {
+		return nil
+	}
+	return n.Variables[name]
+}
+
+func (n *Context) SetVariable(name string, value *list.List) {
+	if n.Variables == nil {
+		n.Variables = make(map[string]*list.List)
+	}
+	n.Variables[name] = value
+}
+
+func (n *Context) ChildContext(results *list.List) Context {
+	clone := Context{}
+	err := copier.Copy(&clone, n)
+	if err != nil {
+		log.Error("Error cloning context :(")
+		panic(err)
+	}
+	clone.MatchingNodes = results
+	return clone
+}
+
+func (n *Context) Clone() Context {
+	clone := Context{}
+	err := copier.Copy(&clone, n)
+	if err != nil {
+		log.Error("Error cloning context :(")
+		panic(err)
+	}
+	return clone
+}
+
+func (n *Context) ReadOnlyClone() Context {
+	clone := n.Clone()
+	clone.DontAutoCreate = true
+	return clone
+}

pkg/yqlib/data_tree_navigator.go

@@ -3,16 +3,14 @@ package yqlib
 import (
 	"fmt"
 
-	"container/list"
-
 	logging "gopkg.in/op/go-logging.v1"
 )
 
 type DataTreeNavigator interface {
-	// given a list of CandidateEntities and a expressionNode,
-	// this will process the list against the given expressionNode and return
-	// a new list of matching candidates
-	GetMatchingNodes(matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error)
+	// given the context and a expressionNode,
+	// this will process the against the given expressionNode and return
+	// a new context of matching candidates
+	GetMatchingNodes(context Context, expressionNode *ExpressionNode) (Context, error)
 }
 
 type dataTreeNavigator struct {
@@ -22,22 +20,22 @@ func NewDataTreeNavigator() DataTreeNavigator {
 	return &dataTreeNavigator{}
 }
 
-func (d *dataTreeNavigator) GetMatchingNodes(matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func (d *dataTreeNavigator) GetMatchingNodes(context Context, expressionNode *ExpressionNode) (Context, error) {
 	if expressionNode == nil {
 		log.Debugf("getMatchingNodes - nothing to do")
-		return matchingNodes, nil
+		return context, nil
 	}
 	log.Debugf("Processing Op: %v", expressionNode.Operation.toString())
 	if log.IsEnabledFor(logging.DEBUG) {
-		for el := matchingNodes.Front(); el != nil; el = el.Next() {
+		for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 			log.Debug(NodeToString(el.Value.(*CandidateNode)))
 		}
 	}
 	log.Debug(">>")
 	handler := expressionNode.Operation.OperationType.Handler
 	if handler != nil {
-		return handler(d, matchingNodes, expressionNode)
+		return handler(d, context, expressionNode)
 	}
-	return nil, fmt.Errorf("Unknown operator %v", expressionNode.Operation.OperationType)
+	return Context{}, fmt.Errorf("Unknown operator %v", expressionNode.Operation.OperationType)
 
 }

pkg/yqlib/doc/Add.md

@@ -101,7 +101,7 @@ will output
 - hello
 ```
 
-## Update array (append)
+## Append to array
 Given a sample.yml file of:
 ```yaml
 a:
@@ -127,6 +127,36 @@ b:
   - 4
 ```
 
+## Relative append
+Given a sample.yml file of:
+```yaml
+a:
+  a1:
+    b:
+      - cat
+  a2:
+    b:
+      - dog
+  a3: {}
+```
+then
+```bash
+yq eval '.a[].b += ["mouse"]' sample.yml
+```
+will output
+```yaml
+a:
+  a1:
+    b:
+      - cat
+      - mouse
+  a2:
+    b:
+      - dog
+      - mouse
+  a3: {b: [mouse]}
+```
+
 ## String concatenation
 Given a sample.yml file of:
 ```yaml
@@ -143,22 +173,6 @@ a: catmeow
 b: meow
 ```
 
-## Relative string concatenation
-Given a sample.yml file of:
-```yaml
-a: cat
-b: meow
-```
-then
-```bash
-yq eval '.a += .b' sample.yml
-```
-will output
-```yaml
-a: catmeow
-b: meow
-```
-
 ## Number addition - float
 If the lhs or rhs are floats then the expression will be calculated with floats.
 
@@ -195,18 +209,20 @@ a: 7
 b: 4
 ```
 
-## Increment number
+## Increment numbers
 Given a sample.yml file of:
 ```yaml
 a: 3
+b: 5
 ```
 then
 ```bash
-yq eval '.a += 1' sample.yml
+yq eval '.[] += 1' sample.yml
 ```
 will output
 ```yaml
 a: 4
+b: 6
 ```
 
 ## Add to null

pkg/yqlib/doc/Anchor and Alias Operators.md

@@ -1,8 +1,99 @@
-Use the `alias` and `anchor` operators to read and write yaml aliases and anchors. The `explode` operator normalises a yaml file (dereference aliases and remove anchor names).
+Use the `alias` and `anchor` operators to read and write yaml aliases and anchors. The `explode` operator normalises a yaml file (dereference (or expands) aliases and remove anchor names).
 
 `yq` supports merge aliases (like `<<: *blah`) however this is no longer in the standard yaml spec (1.2) and so `yq` will automatically add the `!!merge` tag to these nodes as it is effectively a custom tag.
 
 
+## Merge one map
+see https://yaml.org/type/merge.html
+
+Given a sample.yml file of:
+```yaml
+- &CENTER
+  x: 1
+  y: 2
+- &LEFT
+  x: 0
+  y: 2
+- &BIG
+  r: 10
+- &SMALL
+  r: 1
+- !!merge <<: *CENTER
+  r: 10
+```
+then
+```bash
+yq eval '.[4] | explode(.)' sample.yml
+```
+will output
+```yaml
+x: 1
+y: 2
+r: 10
+```
+
+## Merge multiple maps
+see https://yaml.org/type/merge.html
+
+Given a sample.yml file of:
+```yaml
+- &CENTER
+  x: 1
+  y: 2
+- &LEFT
+  x: 0
+  y: 2
+- &BIG
+  r: 10
+- &SMALL
+  r: 1
+- !!merge <<:
+    - *CENTER
+    - *BIG
+```
+then
+```bash
+yq eval '.[4] | explode(.)' sample.yml
+```
+will output
+```yaml
+r: 10
+x: 1
+y: 2
+```
+
+## Override
+see https://yaml.org/type/merge.html
+
+Given a sample.yml file of:
+```yaml
+- &CENTER
+  x: 1
+  y: 2
+- &LEFT
+  x: 0
+  y: 2
+- &BIG
+  r: 10
+- &SMALL
+  r: 1
+- !!merge <<:
+    - *BIG
+    - *LEFT
+    - *SMALL
+  x: 1
+```
+then
+```bash
+yq eval '.[4] | explode(.)' sample.yml
+```
+will output
+```yaml
+r: 10
+x: 1
+y: 2
+```
+
 ## Get anchor
 Given a sample.yml file of:
 ```yaml
@@ -78,6 +169,22 @@ b: &meow purr
 a: *meow
 ```
 
+## Set alias to blank does nothing
+Given a sample.yml file of:
+```yaml
+b: &meow purr
+a: cat
+```
+then
+```bash
+yq eval '.a alias = ""' sample.yml
+```
+will output
+```yaml
+b: &meow purr
+a: cat
+```
+
 ## Set alias relatively using assign-update
 Given a sample.yml file of:
 ```yaml
@@ -183,9 +290,9 @@ bar:
   c: bar_c
 foobarList:
   b: bar_b
-  a: foo_a
-  thing: bar_thing
+  thing: foo_thing
   c: foobarList_c
+  a: foo_a
 foobar:
   c: foo_c
   a: foo_a

pkg/yqlib/doc/Assign (Update).md

@@ -1,10 +1,4 @@
-This operator is used to update node values. It can be used in either the:
 
-### plain form: `=`
-Which will assign the LHS node values to the RHS node values. The RHS expression is run against the matching nodes in the pipeline.
-
-### relative form: `|=`
-This will do a similar thing to the plain form, however, the RHS expression is run against _the LHS nodes_. This is useful for updating values based on old values, e.g. increment.
 ## Create yaml file
 Running
 ```bash

pkg/yqlib/doc/Boolean Operators.md

@@ -1,5 +1,14 @@
-The `or` and `and` operators take two parameters and return a boolean result. `not` flips a boolean from true to false, or vice versa. These are most commonly used with the `select` operator to filter particular nodes.
-## OR example
+The `or` and `and` operators take two parameters and return a boolean result. 
+
+`not` flips a boolean from true to false, or vice versa. 
+
+`any` will return `true` if there are any `true` values in a array sequence, and `all` will return true if _all_ elements in an array are true.
+
+`any_c(condition)` and `all_c(condition)` are like `any` and `all` but they take a condition expression that is used against each element to determine if it's `true`. Note: in `jq` you can simply pass a condition to `any` or `all` and it simply works - `yq` isn't that clever..yet
+
+These are most commonly used with the `select` operator to filter particular nodes.
+
+## `or` example
 Running
 ```bash
 yq eval --null-input 'true or false'
@@ -9,7 +18,7 @@ will output
 true
 ```
 
-## AND example
+## `and` example
 Running
 ```bash
 yq eval --null-input 'true and false'
@@ -41,6 +50,104 @@ will output
   b: fly
 ```
 
+## `any` returns true if any boolean in a given array is true
+Given a sample.yml file of:
+```yaml
+- false
+- true
+```
+then
+```bash
+yq eval 'any' sample.yml
+```
+will output
+```yaml
+true
+```
+
+## `any` returns false for an empty array
+Given a sample.yml file of:
+```yaml
+[]
+```
+then
+```bash
+yq eval 'any' sample.yml
+```
+will output
+```yaml
+false
+```
+
+## `any_c` returns true if any element in the array is true for the given condition.
+Given a sample.yml file of:
+```yaml
+a:
+  - rad
+  - awesome
+b:
+  - meh
+  - whatever
+```
+then
+```bash
+yq eval '.[] |= any_c(. == "awesome")' sample.yml
+```
+will output
+```yaml
+a: true
+b: false
+```
+
+## `all` returns true if all booleans in a given array are true
+Given a sample.yml file of:
+```yaml
+- true
+- true
+```
+then
+```bash
+yq eval 'all' sample.yml
+```
+will output
+```yaml
+true
+```
+
+## `all` returns true for an empty array
+Given a sample.yml file of:
+```yaml
+[]
+```
+then
+```bash
+yq eval 'all' sample.yml
+```
+will output
+```yaml
+true
+```
+
+## `all_c` returns true if all elements in the array are true for the given condition.
+Given a sample.yml file of:
+```yaml
+a:
+  - rad
+  - awesome
+b:
+  - meh
+  - 12
+```
+then
+```bash
+yq eval '.[] |= all_c(tag == "!!str")' sample.yml
+```
+will output
+```yaml
+a: true
+b: false
+```
+
 ## Not true is false
 Running
 ```bash

pkg/yqlib/doc/Comment Operators.md

@@ -1,4 +1,15 @@
 Use these comment operators to set or retrieve comments.
+
+Like the `=` and `|=` assign operators, the same syntax applies when updating comments:
+
+
+### plain form: `=`
+This will assign the LHS nodes comments to the expression on the RHS. The RHS is run against the matching nodes in the pipeline
+
+### relative form: `|=` 
+Similar to the plain form, however the RHS evaluates against each matching LHS node! This is useful if you want to set the comments as a relative expression of the node, for instance its value or path.
+
+
 ## Set line comment
 Given a sample.yml file of:
 ```yaml
@@ -77,7 +88,7 @@ a: cat
 b: dog # leave this
 ```
 
-## Remove all comments
+## Remove (strip) all comments
 Note the use of `...` to ensure key nodes are included.
 
 Given a sample.yml file of:

pkg/yqlib/doc/Entries.md

@@ -0,0 +1,100 @@
+Similar to the same named functions in `jq` these functions convert to/from an object and an array of key-value pairs. This is most useful for performing operations on keys of maps.
+## to_entries Map
+Given a sample.yml file of:
+```yaml
+a: 1
+b: 2
+```
+then
+```bash
+yq eval 'to_entries' sample.yml
+```
+will output
+```yaml
+- key: a
+  value: 1
+- key: b
+  value: 2
+```
+
+## to_entries Array
+Given a sample.yml file of:
+```yaml
+- a
+- b
+```
+then
+```bash
+yq eval 'to_entries' sample.yml
+```
+will output
+```yaml
+- key: 0
+  value: a
+- key: 1
+  value: b
+```
+
+## to_entries null
+Given a sample.yml file of:
+```yaml
+null
+```
+then
+```bash
+yq eval 'to_entries' sample.yml
+```
+will output
+```yaml
+```
+
+## from_entries map
+Given a sample.yml file of:
+```yaml
+a: 1
+b: 2
+```
+then
+```bash
+yq eval 'to_entries | from_entries' sample.yml
+```
+will output
+```yaml
+a: 1
+b: 2
+```
+
+## from_entries with numeric key indexes
+from_entries always creates a map, even for numeric keys
+
+Given a sample.yml file of:
+```yaml
+- a
+- b
+```
+then
+```bash
+yq eval 'to_entries | from_entries' sample.yml
+```
+will output
+```yaml
+0: a
+1: b
+```
+
+## Use with_entries to update keys
+Given a sample.yml file of:
+```yaml
+a: 1
+b: 2
+```
+then
+```bash
+yq eval 'with_entries(.key |= "KEY_" + .)' sample.yml
+```
+will output
+```yaml
+KEY_a: 1
+KEY_b: 2
+```
+

pkg/yqlib/doc/Equals.md

@@ -29,6 +29,24 @@ true
 false
 ```
 
+## Don't match string
+Given a sample.yml file of:
+```yaml
+- cat
+- goat
+- dog
+```
+then
+```bash
+yq eval '.[] | (. != "*at")' sample.yml
+```
+will output
+```yaml
+false
+false
+true
+```
+
 ## Match number
 Given a sample.yml file of:
 ```yaml
@@ -47,6 +65,24 @@ true
 false
 ```
 
+## Dont match number
+Given a sample.yml file of:
+```yaml
+- 3
+- 4
+- 5
+```
+then
+```bash
+yq eval '.[] | (. != 4)' sample.yml
+```
+will output
+```yaml
+true
+false
+true
+```
+
 ## Match nulls
 Running
 ```bash
@@ -57,3 +93,31 @@ will output
 true
 ```
 
+## Non exisitant key doesn't equal a value
+Given a sample.yml file of:
+```yaml
+a: frog
+```
+then
+```bash
+yq eval 'select(.b != "thing")' sample.yml
+```
+will output
+```yaml
+a: frog
+```
+
+## Two non existant keys are equal
+Given a sample.yml file of:
+```yaml
+a: frog
+```
+then
+```bash
+yq eval 'select(.b == .c)' sample.yml
+```
+will output
+```yaml
+a: frog
+```
+

pkg/yqlib/doc/File Operators.md

@@ -35,6 +35,26 @@ will output
 0
 ```
 
+## Get file indices of multiple documents
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+And another sample another.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval-all 'fileIndex' sample.yml another.yml
+```
+will output
+```yaml
+0
+---
+1
+```
+
 ## Get file index alias
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/doc/Has.md

@@ -19,6 +19,29 @@ true
 false
 ```
 
+## Select, checking for existence of deep paths
+Simply pipe in parent expressions into `has`
+
+Given a sample.yml file of:
+```yaml
+- a:
+    b:
+      c: cat
+- a:
+    b:
+      d: dog
+```
+then
+```bash
+yq eval '.[] | select(.a.b | has("c"))' sample.yml
+```
+will output
+```yaml
+a:
+  b:
+    c: cat
+```
+
 ## Has array index
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/doc/Length.md

@@ -16,6 +16,20 @@ will output
 3
 ```
 
+## null length
+Given a sample.yml file of:
+```yaml
+a: null
+```
+then
+```bash
+yq eval '.a | length' sample.yml
+```
+will output
+```yaml
+0
+```
+
 ## Map length
 returns number of entries
 

pkg/yqlib/doc/Multiply (Merge).md

@@ -1,19 +1,34 @@
-Like the multiple operator in jq, depending on the operands, this multiply operator will do different things. Currently only objects are supported, which have the effect of merging the RHS into the LHS.
+Like the multiple operator in jq, depending on the operands, this multiply operator will do different things. Currently numbers, arrays and objects are supported.
 
-To concatenate arrays when merging objects, use the *+ form (see examples below). This will recursively merge objects, appending arrays when it encounters them.
+## Objects and arrays - merging
+Objects are merged deeply matching on matching keys. By default, array values override and are not deeply merged.
 
-To merge only existing fields, use the *? form. Note that this can be used with the concatenate arrays too *+?.
 Note that when merging objects, this operator returns the merged object (not the parent). This will be clearer in the examples below.
 
-Multiplication of strings and numbers are not yet supported.
+### Merge Flags
+You can control how objects are merged by using one or more of the following flags. Multiple flags can be used together, e.g. `.a *+? .b`.  See examples below
 
-## Merging files
+- `+` to append arrays
+- `?` to only merge existing fields
+- `d` to deeply merge arrays
+
+### Merging files
 Note the use of `eval-all` to ensure all documents are loaded into memory.
 
 ```bash
 yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)' file1.yaml file2.yaml
 ```
 
+## Multiply integers
+Running
+```bash
+yq eval --null-input '3 * 4'
+```
+will output
+```yaml
+12
+```
+
 ## Merge objects together, returning merged result only
 Given a sample.yml file of:
 ```yaml
@@ -71,7 +86,6 @@ Given a sample.yml file of:
 a: {things: great}
 b:
   also: "me"
-
 ```
 then
 ```bash
@@ -191,6 +205,72 @@ thing:
   - 4
 ```
 
+## Merge, deeply merging arrays
+Merging arrays deeply means arrays are merge like objects, with indexes as their key. In this case, we merge the first item in the array, and do nothing with the second.
+
+Given a sample.yml file of:
+```yaml
+a:
+  - name: fred
+    age: 12
+  - name: bob
+    age: 32
+b:
+  - name: fred
+    age: 34
+```
+then
+```bash
+yq eval '.a *d .b' sample.yml
+```
+will output
+```yaml
+- name: fred
+  age: 34
+- name: bob
+  age: 32
+```
+
+## Merge arrays of objects together, matching on a key
+It's a complex command, the trickyness comes from needing to have the right context in the expressions.
+First we save the second array into a variable '$two' which lets us reference it later.
+We then need to update the first array. We will use the relative update (|=) because we need to update relative to the current element of the array in the LHS in the RHS expression. 
+We set the current element of the first array as $cur. Now we multiply (merge) $cur with the matching entry in $two, by passing $two through a select filter.
+
+
+Given a sample.yml file of:
+```yaml
+- a: apple
+  b: appleB
+- a: kiwi
+  b: kiwiB
+- a: banana
+  b: bananaB
+```
+And another sample another.yml file of:
+```yaml
+- a: banana
+  c: bananaC
+- a: apple
+  b: appleB2
+- a: dingo
+  c: dingoC
+```
+then
+```bash
+yq eval-all '(select(fi==1) | .[]) as $two | select(fi==0) | .[] |= (. as $cur |  $cur * ($two | select(.a == $cur.a)))' sample.yml another.yml
+```
+will output
+```yaml
+- a: apple
+  b: appleB2
+- a: kiwi
+  b: kiwiB
+- a: banana
+  b: bananaB
+  c: bananaC
+```
+
 ## Merge to prefix an element
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/doc/Reduce.md

@@ -0,0 +1,76 @@
+Reduce is a powerful way to process a collection of data into a new form.
+
+```
+<exp> as $<name> ireduce (<init>; <block>)
+```
+
+e.g.
+
+```
+.[] as $item ireduce (0; . + $item)
+```
+
+On the LHS we are configuring the collection of items that will be reduced `<exp>` as well as what each element will be called `$<name>`. Note that the array has been splatted into its individual elements.
+
+On the RHS there is `<init>`, the starting value of the accumulator and `<block>`, the expression that will update the accumulator for each element in the collection. Note that within the block expression, `.` will evaluate to the current value of the accumulator. 
+
+## yq vs jq syntax
+Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn't as sophisticated as `jq` and its only supports infix notation (e.g. a + b, where the operator is in the middle of the two parameters) - where as `jq` uses a mix of infix notation with _prefix_ notation (e.g. `reduce a b` is like writing `+ a b`).
+
+To that end, the reduce operator is called `ireduce` for backwards compatability if a `jq` like prefix version of `reduce` is ever added.
+
+
+## Sum numbers
+Given a sample.yml file of:
+```yaml
+- 10
+- 2
+- 5
+- 3
+```
+then
+```bash
+yq eval '.[] as $item ireduce (0; . + $item)' sample.yml
+```
+will output
+```yaml
+20
+```
+
+## Merge all yaml files together
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+And another sample another.yml file of:
+```yaml
+b: dog
+```
+then
+```bash
+yq eval-all '. as $item ireduce ({}; . * $item )' sample.yml another.yml
+```
+will output
+```yaml
+a: cat
+b: dog
+```
+
+## Convert an array to an object
+Given a sample.yml file of:
+```yaml
+- name: Cathy
+  has: apples
+- name: Bob
+  has: bananas
+```
+then
+```bash
+yq eval '.[] as $item ireduce ({}; .[$item | .name] = ($item | .has) )' sample.yml
+```
+will output
+```yaml
+Cathy: apples
+Bob: bananas
+```
+

pkg/yqlib/doc/String Operators.md

@@ -18,6 +18,42 @@ will output
 cat; meow; 1; ; true
 ```
 
+## Substitute / Replace string
+This uses golang regex, described [here](https://github.com/google/re2/wiki/Syntax)
+Note the use of `|=` to run in context of the current string value.
+
+Given a sample.yml file of:
+```yaml
+a: dogs are great
+```
+then
+```bash
+yq eval '.a |= sub("dogs", "cats")' sample.yml
+```
+will output
+```yaml
+a: cats are great
+```
+
+## Substitute / Replace string with regex
+This uses golang regex, described [here](https://github.com/google/re2/wiki/Syntax)
+Note the use of `|=` to run in context of the current string value.
+
+Given a sample.yml file of:
+```yaml
+a: cat
+b: heat
+```
+then
+```bash
+yq eval '.[] |= sub("(a)", "${1}r")' sample.yml
+```
+will output
+```yaml
+a: cart
+b: heart
+```
+
 ## Split strings
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/doc/Style.md

@@ -1,4 +1,42 @@
 The style operator can be used to get or set the style of nodes (e.g. string style, yaml style)
+## Update and set style of a particular node (simple)
+Given a sample.yml file of:
+```yaml
+a:
+  b: thing
+  c: something
+```
+then
+```bash
+yq eval '.a.b = "new" | .a.b style="double"' sample.yml
+```
+will output
+```yaml
+a:
+  b: "new"
+  c: something
+```
+
+## Update and set style of a particular node using path variables
+You can use a variable to re-use a path
+
+Given a sample.yml file of:
+```yaml
+a:
+  b: thing
+  c: something
+```
+then
+```bash
+yq eval '.a.b as $x | $x = "new" | $x style="double"' sample.yml
+```
+will output
+```yaml
+a:
+  b: "new"
+  c: something
+```
+
 ## Set tagged style
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/doc/Subtract.md

@@ -0,0 +1,71 @@
+
+## Number subtraction - float
+If the lhs or rhs are floats then the expression will be calculated with floats.
+
+Given a sample.yml file of:
+```yaml
+a: 3
+b: 4.5
+```
+then
+```bash
+yq eval '.a = .a - .b' sample.yml
+```
+will output
+```yaml
+a: -1.5
+b: 4.5
+```
+
+## Number subtraction - float
+If the lhs or rhs are floats then the expression will be calculated with floats.
+
+Given a sample.yml file of:
+```yaml
+a: 3
+b: 4.5
+```
+then
+```bash
+yq eval '.a = .a - .b' sample.yml
+```
+will output
+```yaml
+a: -1.5
+b: 4.5
+```
+
+## Number subtraction - int
+If both the lhs and rhs are ints then the expression will be calculated with ints.
+
+Given a sample.yml file of:
+```yaml
+a: 3
+b: 4
+```
+then
+```bash
+yq eval '.a = .a - .b' sample.yml
+```
+will output
+```yaml
+a: -1
+b: 4
+```
+
+## Decrement numbers
+Given a sample.yml file of:
+```yaml
+a: 3
+b: 5
+```
+then
+```bash
+yq eval '.[] -= 1' sample.yml
+```
+will output
+```yaml
+a: 2
+b: 4
+```
+

pkg/yqlib/doc/Traverse (Read).md

@@ -32,8 +32,23 @@ b: apple
 c: banana
 ```
 
+## Optional Splat
+Just like splat, but won't error if you run it against scalars
+
+Given a sample.yml file of:
+```yaml
+cat
+```
+then
+```bash
+yq eval '.[]' sample.yml
+```
+will output
+```yaml
+```
+
 ## Special characters
-Use quotes around path elements with special characters
+Use quotes with brackets around path elements with special characters
 
 Given a sample.yml file of:
 ```yaml
@@ -41,7 +56,23 @@ Given a sample.yml file of:
 ```
 then
 ```bash
-yq eval '."{}"' sample.yml
+yq eval '.["{}"]' sample.yml
+```
+will output
+```yaml
+frog
+```
+
+## Keys with spaces
+Use quotes with brackets around path elements with special characters
+
+Given a sample.yml file of:
+```yaml
+"red rabbit": frog
+```
+then
+```bash
+yq eval '.["red rabbit"]' sample.yml
 ```
 will output
 ```yaml
@@ -82,6 +113,23 @@ will output
 null
 ```
 
+## Optional identifier
+Like jq, does not output an error when the yaml is not an array or object as expected
+
+Given a sample.yml file of:
+```yaml
+- 1
+- 2
+- 3
+```
+then
+```bash
+yq eval '.a?' sample.yml
+```
+will output
+```yaml
+```
+
 ## Wildcard matching
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/doc/Unique.md

@@ -0,0 +1,82 @@
+This is used to filter out duplicated items in an array.
+
+## Unique array of scalars (string/numbers)
+Given a sample.yml file of:
+```yaml
+- 1
+- 2
+- 3
+- 2
+```
+then
+```bash
+yq eval 'unique' sample.yml
+```
+will output
+```yaml
+- 1
+- 2
+- 3
+```
+
+## Unique nulls
+Unique works on the node value, so it considers different representations of nulls to be different
+
+Given a sample.yml file of:
+```yaml
+- ~
+- null
+- ~
+- null
+```
+then
+```bash
+yq eval 'unique' sample.yml
+```
+will output
+```yaml
+- ~
+- null
+```
+
+## Unique all nulls
+Run against the node tag to unique all the nulls
+
+Given a sample.yml file of:
+```yaml
+- ~
+- null
+- ~
+- null
+```
+then
+```bash
+yq eval 'unique_by(tag)' sample.yml
+```
+will output
+```yaml
+- ~
+```
+
+## Unique array object fields
+Given a sample.yml file of:
+```yaml
+- name: harry
+  pet: cat
+- name: billy
+  pet: dog
+- name: harry
+  pet: dog
+```
+then
+```bash
+yq eval 'unique_by(.name)' sample.yml
+```
+will output
+```yaml
+- name: harry
+  pet: cat
+- name: billy
+  pet: dog
+```
+

pkg/yqlib/doc/Variable Operators.md

@@ -0,0 +1,58 @@
+For more complex scenarios, variables can be used to hold values of expression to be used in other expressions.
+
+## Single value variable
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+then
+```bash
+yq eval '.a as $foo | $foo' sample.yml
+```
+will output
+```yaml
+cat
+```
+
+## Multi value variable
+Given a sample.yml file of:
+```yaml
+- cat
+- dog
+```
+then
+```bash
+yq eval '.[] as $foo | $foo' sample.yml
+```
+will output
+```yaml
+cat
+dog
+```
+
+## Using variables as a lookup
+Example taken from [jq](https://stedolan.github.io/jq/manual/#Variable/SymbolicBindingOperator:...as$identifier|...)
+
+Given a sample.yml file of:
+```yaml
+"posts":
+  - "title": Frist psot
+    "author": anon
+  - "title": A well-written article
+    "author": person1
+"realnames":
+  "anon": Anonymous Coward
+  "person1": Person McPherson
+```
+then
+```bash
+yq eval '.realnames as $names | .posts[] | {"title":.title, "author": $names[.author]}' sample.yml
+```
+will output
+```yaml
+title: Frist psot
+author: Anonymous Coward
+title: A well-written article
+author: Person McPherson
+```
+

pkg/yqlib/doc/aa.md

@@ -1,12 +1,116 @@
 # Operators
 
-In `yq` expressions are made up of operators. Operators have 0-2 arguments and run against the current 'matching' nodes in the expression tree.
+In `yq` expressions are made up of operators and pipes. A context of nodes is passed through the expression and each operation takes the context as input and returns a new context as output. That output is piped in as input for the next operation in the expression. To begin with, the context is set to the first yaml document of the first yaml file (if processing in sequence using eval).
 
 Lets look at a couple of examples.
 
-The `length` operator take no arguments, and will simply return the length of _each_ matching node. So if there were 2 nodes, one string and one array, length will update the 'matching' nodes context to be two new numeric scalar nodes representing the lengths of the orignal 'matching' nodes.
+## Example with a simple operator
 
-The `=` operator takes two arguments, a `lhs` expression and `rhs` expression. It runs the 'matching' nodes context against the `lhs` expression to find the nodes to update, lets call it `lhsNodes`, and then runs the matching nodes against the `rhs` to find the new values, lets call that `rhsNodes`. It updates the `lhsNodes` values with the `rhsNodes` values and _returns the original matching nodes_. This is important, where length changed the matching nodes to be new nodes with the length values, `=` returns the original matching nodes, albeit with some of the nodes values updated. So `.a = 3` will still return the parent matching node, but with the matching child updated.
+Given a document like:
 
-Please see the individual operator docs for more information and examples.
+```yaml
+- [a]
+- "cat"
+```
 
+with an expression:
+
+```
+.[] | length
+```
+
+`yq` will initially set the context as single node of the entire yaml document, an array of two elements.
+
+```yaml
+- [a]
+- "cat"
+```
+
+This gets piped into the splat operator `.[]` which will split out the context into a collection of two nodes `[a]` and `"cat"`. Note that this is _not_ a yaml array.
+
+The `length` operator take no arguments, and will simply return the length of _each_ matching node in the context. So for the context of `[a]` and `"cat"`, it will return a new context of `1` and `3`.
+
+This being the last operation in the expression, the results will be printed out:
+
+```
+1
+3
+```
+
+# Example with an operator that takes arguments.
+
+Given a document like:
+
+```yaml
+a: cat
+b: dog
+```
+
+with an expression:
+
+```
+.a = .b
+```
+
+The `=` operator takes two arguments, a `lhs` expression, which in this case is `.a` and `rhs` expression which is `.b`. 
+
+It pipes the current, lets call it 'root' context through the `lhs` expression of `.a` to return the node 
+
+```yaml
+cat
+```
+
+Note that this node holds not only its value 'cat', but comments and metadata too, including path and parent information.
+
+The `=` operator then pipes the 'root' context through the `rhs` expression of `.b` to return the node
+
+```yaml
+dog
+```
+
+Both sides have now been evaluated, so now the operator copies across the value from the RHS to the value on the LHS, and it returns the now updated context:
+
+```yaml
+a: dog
+b: dog
+```
+
+# Relative update (e.g. `|=`)
+There is another form of the `=` operator which we call the relative form. It's very similar to `=` but with one key difference when evaluating the RHS expression.
+
+In the plain form, we pass in the 'root' level context to the RHS expression. In relative form, we pass in _each result of the LHS_ to the RHS expression. Let's go through an example.
+
+Given a document like:
+
+```yaml
+a: 1
+b: thing
+```
+
+with an expression:
+
+```
+.a |= . + 1
+```
+
+Similar to the `=` operator, `|=` takes two operands, the LHS and RHS.
+
+It pipes the current context (the whole document) through the LHS expression  of `.a` to get the node value:
+
+```
+1
+```
+
+Now it pipes _that LHS context_ into the RHS expression `. + 1` (whereas in the `=` plain form it piped the original document context into the RHS) to yield:
+
+
+```
+2
+```
+
+The assignment operator then copies across the value from the RHS to the value on the LHS, and it returns the now updated 'root' context:
+
+```yaml
+a: 2
+b: thing
+```

pkg/yqlib/doc/headers/Anchor and Alias Operators.md

@@ -1,4 +1,4 @@
-Use the `alias` and `anchor` operators to read and write yaml aliases and anchors. The `explode` operator normalises a yaml file (dereference aliases and remove anchor names).
+Use the `alias` and `anchor` operators to read and write yaml aliases and anchors. The `explode` operator normalises a yaml file (dereference (or expands) aliases and remove anchor names).
 
 `yq` supports merge aliases (like `<<: *blah`) however this is no longer in the standard yaml spec (1.2) and so `yq` will automatically add the `!!merge` tag to these nodes as it is effectively a custom tag.
 

pkg/yqlib/doc/headers/Boolean Operators.md

@@ -1 +1,9 @@
-The `or` and `and` operators take two parameters and return a boolean result. `not` flips a boolean from true to false, or vice versa. These are most commonly used with the `select` operator to filter particular nodes.
+The `or` and `and` operators take two parameters and return a boolean result. 
+
+`not` flips a boolean from true to false, or vice versa. 
+
+`any` will return `true` if there are any `true` values in a array sequence, and `all` will return true if _all_ elements in an array are true.
+
+`any_c(condition)` and `all_c(condition)` are like `any` and `all` but they take a condition expression that is used against each element to determine if it's `true`. Note: in `jq` you can simply pass a condition to `any` or `all` and it simply works - `yq` isn't that clever..yet
+
+These are most commonly used with the `select` operator to filter particular nodes.

pkg/yqlib/doc/headers/Comment Operators.md

@@ -1 +1,11 @@
-Use these comment operators to set or retrieve comments.
+Use these comment operators to set or retrieve comments.
+
+Like the `=` and `|=` assign operators, the same syntax applies when updating comments:
+
+
+### plain form: `=`
+This will assign the LHS nodes comments to the expression on the RHS. The RHS is run against the matching nodes in the pipeline
+
+### relative form: `|=` 
+Similar to the plain form, however the RHS evaluates against each matching LHS node! This is useful if you want to set the comments as a relative expression of the node, for instance its value or path.
+

pkg/yqlib/doc/headers/Entries.md

@@ -0,0 +1 @@
+Similar to the same named functions in `jq` these functions convert to/from an object and an array of key-value pairs. This is most useful for performing operations on keys of maps.

pkg/yqlib/doc/headers/Multiply (Merge).md

@@ -0,0 +1,20 @@
+Like the multiple operator in jq, depending on the operands, this multiply operator will do different things. Currently numbers, arrays and objects are supported.
+
+## Objects and arrays - merging
+Objects are merged deeply matching on matching keys. By default, array values override and are not deeply merged.
+
+Note that when merging objects, this operator returns the merged object (not the parent). This will be clearer in the examples below.
+
+### Merge Flags
+You can control how objects are merged by using one or more of the following flags. Multiple flags can be used together, e.g. `.a *+? .b`.  See examples below
+
+- `+` to append arrays
+- `?` to only merge existing fields
+- `d` to deeply merge arrays
+
+### Merging files
+Note the use of `eval-all` to ensure all documents are loaded into memory.
+
+```bash
+yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)' file1.yaml file2.yaml
+```

pkg/yqlib/doc/headers/Multiply.md

@@ -1,15 +0,0 @@
-Like the multiple operator in jq, depending on the operands, this multiply operator will do different things. Currently only objects are supported, which have the effect of merging the RHS into the LHS.
-
-To concatenate arrays when merging objects, use the *+ form (see examples below). This will recursively merge objects, appending arrays when it encounters them.
-
-To merge only existing fields, use the *? form. Note that this can be used with the concatenate arrays too *+?.
-Note that when merging objects, this operator returns the merged object (not the parent). This will be clearer in the examples below.
-
-Multiplication of strings and numbers are not yet supported.
-
-## Merging files
-Note the use of `eval-all` to ensure all documents are loaded into memory.
-
-```bash
-yq eval-all 'select(fileIndex == 0) * select(fileIndex == 1)' file1.yaml file2.yaml
-```

pkg/yqlib/doc/headers/Reduce.md

@@ -0,0 +1,21 @@
+Reduce is a powerful way to process a collection of data into a new form.
+
+```
+<exp> as $<name> ireduce (<init>; <block>)
+```
+
+e.g.
+
+```
+.[] as $item ireduce (0; . + $item)
+```
+
+On the LHS we are configuring the collection of items that will be reduced `<exp>` as well as what each element will be called `$<name>`. Note that the array has been splatted into its individual elements.
+
+On the RHS there is `<init>`, the starting value of the accumulator and `<block>`, the expression that will update the accumulator for each element in the collection. Note that within the block expression, `.` will evaluate to the current value of the accumulator. 
+
+## yq vs jq syntax
+Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn't as sophisticated as `jq` and its only supports infix notation (e.g. a + b, where the operator is in the middle of the two parameters) - where as `jq` uses a mix of infix notation with _prefix_ notation (e.g. `reduce a b` is like writing `+ a b`).
+
+To that end, the reduce operator is called `ireduce` for backwards compatability if a `jq` like prefix version of `reduce` is ever added.
+

pkg/yqlib/doc/headers/Unique.md

@@ -0,0 +1 @@
+This is used to filter out duplicated items in an array.

pkg/yqlib/doc/headers/Variable Operators.md

@@ -0,0 +1 @@
+For more complex scenarios, variables can be used to hold values of expression to be used in other expressions.

pkg/yqlib/expression_parser.go

@@ -5,9 +5,6 @@ import (
 	"strings"
 )
 
-var myPathTokeniser = newExpressionTokeniser()
-var myPathPostfixer = newExpressionPostFixer()
-
 type ExpressionNode struct {
 	Operation *Operation
 	Lhs       *ExpressionNode
@@ -19,19 +16,21 @@ type ExpressionParser interface {
 }
 
 type expressionParserImpl struct {
+	pathTokeniser expressionTokeniser
+	pathPostFixer expressionPostFixer
 }
 
 func NewExpressionParser() ExpressionParser {
-	return &expressionParserImpl{}
+	return &expressionParserImpl{newExpressionTokeniser(), newExpressionPostFixer()}
 }
 
 func (p *expressionParserImpl) ParseExpression(expression string) (*ExpressionNode, error) {
-	tokens, err := myPathTokeniser.Tokenise(expression)
+	tokens, err := p.pathTokeniser.Tokenise(expression)
 	if err != nil {
 		return nil, err
 	}
 	var Operations []*Operation
-	Operations, err = myPathPostfixer.ConvertToPostfix(tokens)
+	Operations, err = p.pathPostFixer.ConvertToPostfix(tokens)
 	if err != nil {
 		return nil, err
 	}
@@ -70,7 +69,7 @@ func (p *expressionParserImpl) createExpressionTree(postFixPath []*Operation) (*
 		stack = append(stack, &newNode)
 	}
 	if len(stack) != 1 {
-		return nil, fmt.Errorf("expected end of expression but found '%v', please check expression syntax", strings.TrimSpace(stack[1].Operation.StringValue))
+		return nil, fmt.Errorf("Bad expression, please check expression syntax")
 	}
 	return stack[0], nil
 }

pkg/yqlib/expression_parser_test.go

@@ -38,5 +38,5 @@ func TestPathTreeOneArgForOneArgOp(t *testing.T) {
 
 func TestPathTreeExtraArgs(t *testing.T) {
 	_, err := NewExpressionParser().ParseExpression("sortKeys(.) explode(.)")
-	test.AssertResultComplex(t, "expected end of expression but found 'explode', please check expression syntax", err.Error())
+	test.AssertResultComplex(t, "Bad expression, please check expression syntax", err.Error())
 }

pkg/yqlib/expression_postfix.go

@@ -20,20 +20,22 @@ func newExpressionPostFixer() expressionPostFixer {
 func popOpToResult(opStack []*token, result []*Operation) ([]*token, []*Operation) {
 	var newOp *token
 	opStack, newOp = opStack[0:len(opStack)-1], opStack[len(opStack)-1]
+	log.Debugf("popped %v from opstack to results", newOp.toString(true))
 	return opStack, append(result, newOp.Operation)
 }
 
 func (p *expressionPostFixerImpl) ConvertToPostfix(infixTokens []*token) ([]*Operation, error) {
 	var result []*Operation
-	// surround the whole thing with quotes
-	var opStack = []*token{&token{TokenType: openBracket}}
+	// surround the whole thing with brackets
+	var opStack = []*token{{TokenType: openBracket}}
 	var tokens = append(infixTokens, &token{TokenType: closeBracket})
 
 	for _, currentToken := range tokens {
-		log.Debugf("postfix processing currentToken %v, %v", currentToken.toString(), currentToken.Operation)
+		log.Debugf("postfix processing currentToken %v", currentToken.toString(true))
 		switch currentToken.TokenType {
 		case openBracket, openCollect, openCollectObject:
 			opStack = append(opStack, currentToken)
+			log.Debugf("put %v onto the opstack", currentToken.toString(true))
 		case closeCollect, closeCollectObject:
 			var opener tokenType = openCollect
 			var collectOperator *operationType = collectOpType
@@ -41,23 +43,37 @@ func (p *expressionPostFixerImpl) ConvertToPostfix(infixTokens []*token) ([]*Ope
 				opener = openCollectObject
 				collectOperator = collectObjectOpType
 			}
-			itemsInMiddle := false
+
 			for len(opStack) > 0 && opStack[len(opStack)-1].TokenType != opener {
 				opStack, result = popOpToResult(opStack, result)
-				itemsInMiddle = true
-			}
-			if !itemsInMiddle {
-				// must be an empty collection, add the empty object as a LHS parameter
-				result = append(result, &Operation{OperationType: emptyOpType})
 			}
 			if len(opStack) == 0 {
 				return nil, errors.New("Bad path expression, got close collect brackets without matching opening bracket")
 			}
-			// now we should have [] as the last element on the opStack, get rid of it
+			// now we should have [ as the last element on the opStack, get rid of it
 			opStack = opStack[0 : len(opStack)-1]
-			//and append a collect to the opStack
-			opStack = append(opStack, &token{TokenType: operationToken, Operation: &Operation{OperationType: shortPipeOpType}})
-			opStack = append(opStack, &token{TokenType: operationToken, Operation: &Operation{OperationType: collectOperator}})
+			log.Debugf("deleteing open bracket from opstack")
+
+			//and append a collect to the result
+			// hack - see if there's the optional traverse flag
+			// on the close op - move it to the collect op.
+			// allows for .["cat"]?
+			prefs := traversePreferences{}
+			closeTokenMatch := string(currentToken.Match.Bytes)
+			if closeTokenMatch[len(closeTokenMatch)-1:] == "?" {
+				prefs.OptionalTraverse = true
+			}
+			result = append(result, &Operation{OperationType: collectOperator, Preferences: prefs})
+			log.Debugf("put collect onto the result")
+			result = append(result, &Operation{OperationType: shortPipeOpType})
+			log.Debugf("put shortpipe onto the result")
+
+			//traverseArrayCollect is a sneaky op that needs to be included too
+			//when closing a []
+			if len(opStack) > 0 && opStack[len(opStack)-1].Operation != nil && opStack[len(opStack)-1].Operation.OperationType == traverseArrayOpType {
+				opStack, result = popOpToResult(opStack, result)
+			}
+
 		case closeBracket:
 			for len(opStack) > 0 && opStack[len(opStack)-1].TokenType != openBracket {
 				opStack, result = popOpToResult(opStack, result)
@@ -73,11 +89,12 @@ func (p *expressionPostFixerImpl) ConvertToPostfix(infixTokens []*token) ([]*Ope
 			// pop off higher precedent operators onto the result
 			for len(opStack) > 0 &&
 				opStack[len(opStack)-1].TokenType == operationToken &&
-				opStack[len(opStack)-1].Operation.OperationType.Precedence >= currentPrecedence {
+				opStack[len(opStack)-1].Operation.OperationType.Precedence > currentPrecedence {
 				opStack, result = popOpToResult(opStack, result)
 			}
 			// add this operator to the opStack
 			opStack = append(opStack, currentToken)
+			log.Debugf("put %v onto the opstack", currentToken.toString(true))
 		}
 	}
 

pkg/yqlib/expression_processing_test.go

@@ -12,40 +12,115 @@ var pathTests = []struct {
 	expectedTokens  []interface{}
 	expectedPostFix []interface{}
 }{
+	{
+		`.[0]`,
+		append(make([]interface{}, 0), "SELF", "TRAVERSE_ARRAY", "[", "0 (int64)", "]"),
+		append(make([]interface{}, 0), "SELF", "0 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
+	},
+	{
+		`.[0][1]`,
+		append(make([]interface{}, 0), "SELF", "TRAVERSE_ARRAY", "[", "0 (int64)", "]", "TRAVERSE_ARRAY", "[", "1 (int64)", "]"),
+		append(make([]interface{}, 0), "SELF", "0 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "1 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
+	},
+	{
+		`"\""`,
+		append(make([]interface{}, 0), "\" (string)"),
+		append(make([]interface{}, 0), "\" (string)"),
+	},
+	{
+		`[]|join(".")`,
+		append(make([]interface{}, 0), "[", "EMPTY", "]", "PIPE", "JOIN", "(", ". (string)", ")"),
+		append(make([]interface{}, 0), "EMPTY", "COLLECT", "SHORT_PIPE", ". (string)", "JOIN", "PIPE"),
+	},
+	{
+		`{"cool": .b or .c}`,
+		append(make([]interface{}, 0), "{", "cool (string)", "CREATE_MAP", "b", "OR", "c", "}"),
+		append(make([]interface{}, 0), "cool (string)", "b", "c", "OR", "CREATE_MAP", "COLLECT_OBJECT", "SHORT_PIPE"),
+	},
+	{
+		`{"cool": []|join(".")}`,
+		append(make([]interface{}, 0), "{", "cool (string)", "CREATE_MAP", "[", "EMPTY", "]", "PIPE", "JOIN", "(", ". (string)", ")", "}"),
+		append(make([]interface{}, 0), "cool (string)", "EMPTY", "COLLECT", "SHORT_PIPE", ". (string)", "JOIN", "PIPE", "CREATE_MAP", "COLLECT_OBJECT", "SHORT_PIPE"),
+	},
+	{
+		`.a as $item ireduce (0; . + $item)`, // note - add code to shuffle reduce to this position for postfix
+		append(make([]interface{}, 0), "a", "ASSIGN_VARIABLE", "GET_VARIABLE", "REDUCE", "(", "0 (int64)", "BLOCK", "SELF", "ADD", "GET_VARIABLE", ")"),
+		append(make([]interface{}, 0), "a", "GET_VARIABLE", "ASSIGN_VARIABLE", "0 (int64)", "SELF", "GET_VARIABLE", "ADD", "BLOCK", "REDUCE"),
+	},
+	{
+		`.a | .b | .c`,
+		append(make([]interface{}, 0), "a", "PIPE", "b", "PIPE", "c"),
+		append(make([]interface{}, 0), "a", "b", "c", "PIPE", "PIPE"),
+	},
 	{
 		`[]`,
-		append(make([]interface{}, 0), "[", "]"),
+		append(make([]interface{}, 0), "[", "EMPTY", "]"),
 		append(make([]interface{}, 0), "EMPTY", "COLLECT", "SHORT_PIPE"),
 	},
+	{
+		`{}`,
+		append(make([]interface{}, 0), "{", "EMPTY", "}"),
+		append(make([]interface{}, 0), "EMPTY", "COLLECT_OBJECT", "SHORT_PIPE"),
+	},
+	{
+		`[{}]`,
+		append(make([]interface{}, 0), "[", "{", "EMPTY", "}", "]"),
+		append(make([]interface{}, 0), "EMPTY", "COLLECT_OBJECT", "SHORT_PIPE", "COLLECT", "SHORT_PIPE"),
+	},
+	{
+		`.realnames as $names | $names["anon"]`,
+		append(make([]interface{}, 0), "realnames", "ASSIGN_VARIABLE", "GET_VARIABLE", "PIPE", "GET_VARIABLE", "TRAVERSE_ARRAY", "[", "anon (string)", "]"),
+		append(make([]interface{}, 0), "realnames", "GET_VARIABLE", "ASSIGN_VARIABLE", "GET_VARIABLE", "anon (string)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "PIPE"),
+	},
+	{
+		`.b[.a]`,
+		append(make([]interface{}, 0), "b", "TRAVERSE_ARRAY", "[", "a", "]"),
+		append(make([]interface{}, 0), "b", "a", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
+	},
+	{
+		`.b[.a]?`,
+		append(make([]interface{}, 0), "b", "TRAVERSE_ARRAY", "[", "a", "]"),
+		append(make([]interface{}, 0), "b", "a", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
+	},
 	{
 		`.[]`,
-		append(make([]interface{}, 0), "TRAVERSE_ARRAY", "[", "]"),
-		append(make([]interface{}, 0), "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
+		append(make([]interface{}, 0), "SELF", "TRAVERSE_ARRAY", "[", "EMPTY", "]"),
+		append(make([]interface{}, 0), "SELF", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
 	},
 	{
 		`.a[]`,
-		append(make([]interface{}, 0), "a", "SHORT_PIPE", "TRAVERSE_ARRAY", "[", "]"),
-		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE"),
+		append(make([]interface{}, 0), "a", "TRAVERSE_ARRAY", "[", "EMPTY", "]"),
+		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
+	},
+	{
+		`.a[]?`,
+		append(make([]interface{}, 0), "a", "TRAVERSE_ARRAY", "[", "EMPTY", "]"),
+		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
 	},
 	{
 		`.a.[]`,
-		append(make([]interface{}, 0), "a", "SHORT_PIPE", "TRAVERSE_ARRAY", "[", "]"),
-		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE"),
+		append(make([]interface{}, 0), "a", "TRAVERSE_ARRAY", "[", "EMPTY", "]"),
+		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
 	},
 	{
 		`.a[0]`,
-		append(make([]interface{}, 0), "a", "SHORT_PIPE", "TRAVERSE_ARRAY", "[", "0 (int64)", "]"),
-		append(make([]interface{}, 0), "a", "0 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE"),
+		append(make([]interface{}, 0), "a", "TRAVERSE_ARRAY", "[", "0 (int64)", "]"),
+		append(make([]interface{}, 0), "a", "0 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
+	},
+	{
+		`.a[0]?`,
+		append(make([]interface{}, 0), "a", "TRAVERSE_ARRAY", "[", "0 (int64)", "]"),
+		append(make([]interface{}, 0), "a", "0 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
 	},
 	{
 		`.a.[0]`,
-		append(make([]interface{}, 0), "a", "SHORT_PIPE", "TRAVERSE_ARRAY", "[", "0 (int64)", "]"),
-		append(make([]interface{}, 0), "a", "0 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE"),
+		append(make([]interface{}, 0), "a", "TRAVERSE_ARRAY", "[", "0 (int64)", "]"),
+		append(make([]interface{}, 0), "a", "0 (int64)", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY"),
 	},
 	{
 		`.a[].c`,
-		append(make([]interface{}, 0), "a", "SHORT_PIPE", "TRAVERSE_ARRAY", "[", "]", "SHORT_PIPE", "c"),
-		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE", "c", "SHORT_PIPE"),
+		append(make([]interface{}, 0), "a", "TRAVERSE_ARRAY", "[", "EMPTY", "]", "SHORT_PIPE", "c"),
+		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "c", "SHORT_PIPE"),
 	},
 	{
 		`[3]`,
@@ -69,18 +144,18 @@ var pathTests = []struct {
 	},
 	{
 		`.a | .[].b == "apple"`,
-		append(make([]interface{}, 0), "a", "PIPE", "TRAVERSE_ARRAY", "[", "]", "SHORT_PIPE", "b", "EQUALS", "apple (string)"),
-		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "b", "SHORT_PIPE", "apple (string)", "EQUALS", "PIPE"),
+		append(make([]interface{}, 0), "a", "PIPE", "SELF", "TRAVERSE_ARRAY", "[", "EMPTY", "]", "SHORT_PIPE", "b", "EQUALS", "apple (string)"),
+		append(make([]interface{}, 0), "a", "SELF", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "b", "SHORT_PIPE", "apple (string)", "EQUALS", "PIPE"),
 	},
 	{
 		`(.a | .[].b) == "apple"`,
-		append(make([]interface{}, 0), "(", "a", "PIPE", "TRAVERSE_ARRAY", "[", "]", "SHORT_PIPE", "b", ")", "EQUALS", "apple (string)"),
-		append(make([]interface{}, 0), "a", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "b", "SHORT_PIPE", "PIPE", "apple (string)", "EQUALS"),
+		append(make([]interface{}, 0), "(", "a", "PIPE", "SELF", "TRAVERSE_ARRAY", "[", "EMPTY", "]", "SHORT_PIPE", "b", ")", "EQUALS", "apple (string)"),
+		append(make([]interface{}, 0), "a", "SELF", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "b", "SHORT_PIPE", "PIPE", "apple (string)", "EQUALS"),
 	},
 	{
 		`.[] | select(. == "*at")`,
-		append(make([]interface{}, 0), "TRAVERSE_ARRAY", "[", "]", "PIPE", "SELECT", "(", "SELF", "EQUALS", "*at (string)", ")"),
-		append(make([]interface{}, 0), "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SELF", "*at (string)", "EQUALS", "SELECT", "PIPE"),
+		append(make([]interface{}, 0), "SELF", "TRAVERSE_ARRAY", "[", "EMPTY", "]", "PIPE", "SELECT", "(", "SELF", "EQUALS", "*at (string)", ")"),
+		append(make([]interface{}, 0), "SELF", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SELF", "*at (string)", "EQUALS", "SELECT", "PIPE"),
 	},
 	{
 		`[true]`,
@@ -113,9 +188,9 @@ var pathTests = []struct {
 		append(make([]interface{}, 0), "a", "mike (string)", "CREATE_MAP", "COLLECT_OBJECT", "SHORT_PIPE"),
 	},
 	{
-		`{.a: .c, .b.[]: .f.g.[]}`,
-		append(make([]interface{}, 0), "{", "a", "CREATE_MAP", "c", "UNION", "b", "SHORT_PIPE", "TRAVERSE_ARRAY", "[", "]", "CREATE_MAP", "f", "SHORT_PIPE", "g", "SHORT_PIPE", "TRAVERSE_ARRAY", "[", "]", "}"),
-		append(make([]interface{}, 0), "a", "c", "CREATE_MAP", "b", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE", "f", "g", "SHORT_PIPE", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE", "CREATE_MAP", "UNION", "COLLECT_OBJECT", "SHORT_PIPE"),
+		`{.a: .c, .b.[]: .f.g[]}`,
+		append(make([]interface{}, 0), "{", "a", "CREATE_MAP", "c", "UNION", "b", "TRAVERSE_ARRAY", "[", "EMPTY", "]", "CREATE_MAP", "f", "SHORT_PIPE", "g", "TRAVERSE_ARRAY", "[", "EMPTY", "]", "}"),
+		append(make([]interface{}, 0), "a", "c", "CREATE_MAP", "b", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "f", "g", "EMPTY", "COLLECT", "SHORT_PIPE", "TRAVERSE_ARRAY", "SHORT_PIPE", "CREATE_MAP", "UNION", "COLLECT_OBJECT", "SHORT_PIPE"),
 	},
 	{
 		`explode(.a.b)`,
@@ -167,11 +242,6 @@ var pathTests = []struct {
 		append(make([]interface{}, 0), "foo*", "PIPE", "(", "SELF", "ASSIGN_STYLE", "flow (string)", ")"),
 		append(make([]interface{}, 0), "foo*", "SELF", "flow (string)", "ASSIGN_STYLE", "PIPE"),
 	},
-	{
-		`{}`,
-		append(make([]interface{}, 0), "{", "}"),
-		append(make([]interface{}, 0), "EMPTY", "COLLECT_OBJECT", "SHORT_PIPE"),
-	},
 }
 
 var tokeniser = newExpressionTokeniser()
@@ -185,7 +255,7 @@ func TestPathParsing(t *testing.T) {
 		}
 		var tokenValues []interface{}
 		for _, token := range tokens {
-			tokenValues = append(tokenValues, token.toString())
+			tokenValues = append(tokenValues, token.toString(false))
 		}
 		test.AssertResultComplexWithContext(t, tt.expectedTokens, tokenValues, fmt.Sprintf("tokenise: %v", tt.path))
 

pkg/yqlib/expression_tokeniser.go

@@ -29,14 +29,17 @@ const (
 type token struct {
 	TokenType            tokenType
 	Operation            *Operation
-	AssignOperation      *Operation // e.g. tag (GetTag) op becomes AssignTag if '=' follows it
-	CheckForPostTraverse bool       // e.g. [1]cat should really be [1].cat
+	AssignOperation      *Operation      // e.g. tag (GetTag) op becomes AssignTag if '=' follows it
+	CheckForPostTraverse bool            // e.g. [1]cat should really be [1].cat
+	Match                *machines.Match // match that created this token
 
 }
 
-func (t *token) toString() string {
+func (t *token) toString(detail bool) string {
 	if t.TokenType == operationToken {
-		log.Debug("toString, its an op")
+		if detail {
+			return fmt.Sprintf("%v (%v)", t.Operation.toString(), t.Operation.OperationType.Precedence)
+		}
 		return t.Operation.toString()
 	} else if t.TokenType == openBracket {
 		return "("
@@ -61,12 +64,19 @@ func (t *token) toString() string {
 func pathToken(wrapped bool) lex.Action {
 	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
 		value := string(m.Bytes)
+		prefs := traversePreferences{}
+
+		if value[len(value)-1:] == "?" {
+			prefs.OptionalTraverse = true
+			value = value[:len(value)-1]
+		}
+
 		value = value[1:]
 		if wrapped {
 			value = unwrap(value)
 		}
 		log.Debug("PathToken %v", value)
-		op := &Operation{OperationType: traversePathOpType, Value: value, StringValue: value, Preferences: traversePreferences{}}
+		op := &Operation{OperationType: traversePathOpType, Value: value, StringValue: value, Preferences: prefs}
 		return &token{TokenType: operationToken, Operation: op, CheckForPostTraverse: true}, nil
 	}
 }
@@ -98,6 +108,9 @@ func multiplyWithPrefs() lex.Action {
 		if strings.Contains(options, "?") {
 			prefs.TraversePrefs = traversePreferences{DontAutoCreate: true}
 		}
+		if strings.Contains(options, "d") {
+			prefs.DeepMergeArrays = true
+		}
 		op := &Operation{OperationType: multiplyOpType, Value: multiplyOpType.Type, StringValue: options, Preferences: prefs}
 		return &token{TokenType: operationToken, Operation: op}, nil
 	}
@@ -133,7 +146,7 @@ func assignAllCommentsOp(updateAssign bool) lex.Action {
 
 func literalToken(pType tokenType, checkForPost bool) lex.Action {
 	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
-		return &token{TokenType: pType, CheckForPostTraverse: checkForPost}, nil
+		return &token{TokenType: pType, CheckForPostTraverse: checkForPost, Match: m}, nil
 	}
 }
 
@@ -176,10 +189,24 @@ func stringValue(wrapped bool) lex.Action {
 		if wrapped {
 			value = unwrap(value)
 		}
+		value = strings.ReplaceAll(value, "\\\"", "\"")
 		return &token{TokenType: operationToken, Operation: createValueOperation(value, value)}, nil
 	}
 }
 
+func getVariableOpToken() lex.Action {
+	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
+		value := string(m.Bytes)
+
+		value = value[1:]
+
+		getVarOperation := createValueOperation(value, value)
+		getVarOperation.OperationType = getVariableOpType
+
+		return &token{TokenType: operationToken, Operation: getVarOperation, CheckForPostTraverse: true}, nil
+	}
+}
+
 func envOp(strenv bool) lex.Action {
 	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
 		value := string(m.Bytes)
@@ -233,10 +260,14 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`sortKeys`), opToken(sortKeysOpType))
 	lexer.Add([]byte(`select`), opToken(selectOpType))
 	lexer.Add([]byte(`has`), opToken(hasOpType))
+	lexer.Add([]byte(`unique`), opToken(uniqueOpType))
+	lexer.Add([]byte(`unique_by`), opToken(uniqueByOpType))
 	lexer.Add([]byte(`explode`), opToken(explodeOpType))
 	lexer.Add([]byte(`or`), opToken(orOpType))
 	lexer.Add([]byte(`and`), opToken(andOpType))
 	lexer.Add([]byte(`not`), opToken(notOpType))
+	lexer.Add([]byte(`ireduce`), opToken(reduceOpType))
+	lexer.Add([]byte(`;`), opToken(blockOpType))
 	lexer.Add([]byte(`\/\/`), opToken(alternativeOpType))
 
 	lexer.Add([]byte(`documentIndex`), opToken(getDocumentIndexOpType))
@@ -244,6 +275,13 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`splitDoc`), opToken(splitDocumentOpType))
 
 	lexer.Add([]byte(`join`), opToken(joinStringOpType))
+	lexer.Add([]byte(`sub`), opToken(subStringOpType))
+
+	lexer.Add([]byte(`any`), opToken(anyOpType))
+	lexer.Add([]byte(`any_c`), opToken(anyConditionOpType))
+	lexer.Add([]byte(`all`), opToken(allOpType))
+	lexer.Add([]byte(`all_c`), opToken(allConditionOpType))
+
 	lexer.Add([]byte(`split`), opToken(splitStringOpType))
 	lexer.Add([]byte(`keys`), opToken(keysOpType))
 
@@ -256,6 +294,9 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`fileIndex`), opToken(getFileIndexOpType))
 	lexer.Add([]byte(`fi`), opToken(getFileIndexOpType))
 	lexer.Add([]byte(`path`), opToken(getPathOpType))
+	lexer.Add([]byte(`to_entries`), opToken(toEntriesOpType))
+	lexer.Add([]byte(`from_entries`), opToken(fromEntriesOpType))
+	lexer.Add([]byte(`with_entries`), opToken(withEntriesOpType))
 
 	lexer.Add([]byte(`lineComment`), opTokenWithPrefs(getCommentOpType, assignCommentOpType, commentOpPreferences{LineComment: true}))
 
@@ -269,6 +310,7 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`collect`), opToken(collectOpType))
 
 	lexer.Add([]byte(`\s*==\s*`), opToken(equalsOpType))
+	lexer.Add([]byte(`\s*!=\s*`), opToken(notEqualsOpType))
 	lexer.Add([]byte(`\s*=\s*`), assignOpToken(false))
 
 	lexer.Add([]byte(`del`), opToken(deleteChildOpType))
@@ -277,8 +319,8 @@ func initLexer() (*lex.Lexer, error) {
 
 	lexer.Add([]byte("( |\t|\n|\r)+"), skip)
 
-	lexer.Add([]byte(`\."[^ "]+"`), pathToken(true))
-	lexer.Add([]byte(`\.[^ \}\{\:\[\],\|\.\[\(\)=]+`), pathToken(false))
+	lexer.Add([]byte(`\."[^ "]+"\??`), pathToken(true))
+	lexer.Add([]byte(`\.[^ \}\{\:\[\],\|\.\[\(\)=]+\??`), pathToken(false))
 	lexer.Add([]byte(`\.`), selfToken())
 
 	lexer.Add([]byte(`\|`), opToken(pipeOpType))
@@ -293,19 +335,23 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`[Nn][Uu][Ll][Ll]`), nullValue())
 	lexer.Add([]byte(`~`), nullValue())
 
-	lexer.Add([]byte(`"[^"]*"`), stringValue(true))
+	lexer.Add([]byte(`"([^"\\]*(\\.[^"\\]*)*)"`), stringValue(true))
 	lexer.Add([]byte(`strenv\([^\)]+\)`), envOp(true))
 	lexer.Add([]byte(`env\([^\)]+\)`), envOp(false))
 
 	lexer.Add([]byte(`\[`), literalToken(openCollect, false))
-	lexer.Add([]byte(`\]`), literalToken(closeCollect, true))
+	lexer.Add([]byte(`\]\??`), literalToken(closeCollect, true))
 	lexer.Add([]byte(`\{`), literalToken(openCollectObject, false))
 	lexer.Add([]byte(`\}`), literalToken(closeCollectObject, true))
-	lexer.Add([]byte(`\*[\+|\?]*`), multiplyWithPrefs())
+	lexer.Add([]byte(`\*[\+|\?d]*`), multiplyWithPrefs())
 	lexer.Add([]byte(`\+`), opToken(addOpType))
 	lexer.Add([]byte(`\+=`), opToken(addAssignOpType))
+	lexer.Add([]byte(`\-`), opToken(subtractOpType))
+	lexer.Add([]byte(`\-=`), opToken(subtractAssignOpType))
+	lexer.Add([]byte(`\$[a-zA-Z_-0-9]+`), getVariableOpToken())
+	lexer.Add([]byte(`as`), opToken(assignVariableOpType))
 
-	err := lexer.Compile()
+	err := lexer.CompileNFA()
 	if err != nil {
 		return nil, err
 	}
@@ -339,7 +385,7 @@ func (p *expressionTokeniserImpl) Tokenise(expression string) ([]*token, error)
 
 		if tok != nil {
 			currentToken := tok.(*token)
-			log.Debugf("Tokenising %v", currentToken.toString())
+			log.Debugf("Tokenising %v", currentToken.toString(true))
 			tokens = append(tokens, currentToken)
 		}
 		if err != nil {
@@ -365,10 +411,19 @@ func (p *expressionTokeniserImpl) handleToken(tokens []*token, index int, postPr
 	skipNextToken = false
 	currentToken := tokens[index]
 
+	log.Debug("processing %v", currentToken.toString(true))
+
 	if currentToken.TokenType == traverseArrayCollect {
 		//need to put a traverse array then a collect currentToken
 		// do this by adding traverse then converting currentToken to collect
 
+		if index == 0 || tokens[index-1].TokenType != operationToken ||
+			tokens[index-1].Operation.OperationType != traversePathOpType {
+			log.Debug("  adding self")
+			op := &Operation{OperationType: selfReferenceOpType, StringValue: "SELF"}
+			postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
+		}
+		log.Debug("  adding traverse array")
 		op := &Operation{OperationType: traverseArrayOpType, StringValue: "TRAVERSE_ARRAY"}
 		postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
 
@@ -379,34 +434,41 @@ func (p *expressionTokeniserImpl) handleToken(tokens []*token, index int, postPr
 	if index != len(tokens)-1 && currentToken.AssignOperation != nil &&
 		tokens[index+1].TokenType == operationToken &&
 		tokens[index+1].Operation.OperationType == assignOpType {
+		log.Debug("  its an update assign")
 		currentToken.Operation = currentToken.AssignOperation
 		currentToken.Operation.UpdateAssign = tokens[index+1].Operation.UpdateAssign
 		skipNextToken = true
 	}
 
+	log.Debug("  adding token to the fixed list")
 	postProcessedTokens = append(postProcessedTokens, currentToken)
 
+	if index != len(tokens)-1 &&
+		((currentToken.TokenType == openCollect && tokens[index+1].TokenType == closeCollect) ||
+			(currentToken.TokenType == openCollectObject && tokens[index+1].TokenType == closeCollectObject)) {
+		log.Debug("  adding empty")
+		op := &Operation{OperationType: emptyOpType, StringValue: "EMPTY"}
+		postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
+	}
+
 	if index != len(tokens)-1 && currentToken.CheckForPostTraverse &&
 		tokens[index+1].TokenType == operationToken &&
 		tokens[index+1].Operation.OperationType == traversePathOpType {
+		log.Debug("  adding pipe because the next thing is traverse")
 		op := &Operation{OperationType: shortPipeOpType, Value: "PIPE"}
 		postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
 	}
 	if index != len(tokens)-1 && currentToken.CheckForPostTraverse &&
 		tokens[index+1].TokenType == openCollect {
 
-		op := &Operation{OperationType: shortPipeOpType, Value: "PIPE"}
+		// if tokens[index].TokenType == closeCollect {
+		// 	log.Debug("  adding pipe because next is opencollect")
+		// 	op := &Operation{OperationType: shortPipeOpType, Value: "PIPE"}
+		// 	postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
+		// }
+		log.Debug("  adding traverArray because next is opencollect")
+		op := &Operation{OperationType: traverseArrayOpType}
 		postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
-
-		op = &Operation{OperationType: traverseArrayOpType}
-		postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
-	}
-	if index != len(tokens)-1 && currentToken.CheckForPostTraverse &&
-		tokens[index+1].TokenType == traverseArrayCollect {
-
-		op := &Operation{OperationType: shortPipeOpType, Value: "PIPE"}
-		postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
-
 	}
 	return postProcessedTokens, skipNextToken
 }

pkg/yqlib/lib.go

@@ -20,11 +20,11 @@ type operationType struct {
 	Handler    operatorHandler
 }
 
-// operators TODO:
-// - mergeEmpty (sets only if the document is empty, do I do that now?)
-
 var orOpType = &operationType{Type: "OR", NumArgs: 2, Precedence: 20, Handler: orOperator}
 var andOpType = &operationType{Type: "AND", NumArgs: 2, Precedence: 20, Handler: andOperator}
+var reduceOpType = &operationType{Type: "REDUCE", NumArgs: 2, Precedence: 35, Handler: reduceOperator}
+
+var blockOpType = &operationType{Type: "BLOCK", Precedence: 10, NumArgs: 2, Handler: emptyOperator}
 
 var unionOpType = &operationType{Type: "UNION", NumArgs: 2, Precedence: 10, Handler: unionOperator}
 
@@ -32,9 +32,11 @@ var pipeOpType = &operationType{Type: "PIPE", NumArgs: 2, Precedence: 30, Handle
 
 var assignOpType = &operationType{Type: "ASSIGN", NumArgs: 2, Precedence: 40, Handler: assignUpdateOperator}
 var addAssignOpType = &operationType{Type: "ADD_ASSIGN", NumArgs: 2, Precedence: 40, Handler: addAssignOperator}
+var subtractAssignOpType = &operationType{Type: "SUBTRACT_ASSIGN", NumArgs: 2, Precedence: 40, Handler: subtractAssignOperator}
 
 var assignAttributesOpType = &operationType{Type: "ASSIGN_ATTRIBUTES", NumArgs: 2, Precedence: 40, Handler: assignAttributesOperator}
 var assignStyleOpType = &operationType{Type: "ASSIGN_STYLE", NumArgs: 2, Precedence: 40, Handler: assignStyleOperator}
+var assignVariableOpType = &operationType{Type: "ASSIGN_VARIABLE", NumArgs: 2, Precedence: 40, Handler: assignVariableOperator}
 var assignTagOpType = &operationType{Type: "ASSIGN_TAG", NumArgs: 2, Precedence: 40, Handler: assignTagOperator}
 var assignCommentOpType = &operationType{Type: "ASSIGN_COMMENT", NumArgs: 2, Precedence: 40, Handler: assignCommentsOperator}
 var assignAnchorOpType = &operationType{Type: "ASSIGN_ANCHOR", NumArgs: 2, Precedence: 40, Handler: assignAnchorOperator}
@@ -42,16 +44,31 @@ var assignAliasOpType = &operationType{Type: "ASSIGN_ALIAS", NumArgs: 2, Precede
 
 var multiplyOpType = &operationType{Type: "MULTIPLY", NumArgs: 2, Precedence: 42, Handler: multiplyOperator}
 var addOpType = &operationType{Type: "ADD", NumArgs: 2, Precedence: 42, Handler: addOperator}
+var subtractOpType = &operationType{Type: "SUBTRACT", NumArgs: 2, Precedence: 42, Handler: subtractOperator}
 var alternativeOpType = &operationType{Type: "ALTERNATIVE", NumArgs: 2, Precedence: 42, Handler: alternativeOperator}
 
 var equalsOpType = &operationType{Type: "EQUALS", NumArgs: 2, Precedence: 40, Handler: equalsOperator}
-var createMapOpType = &operationType{Type: "CREATE_MAP", NumArgs: 2, Precedence: 40, Handler: createMapOperator}
+var notEqualsOpType = &operationType{Type: "EQUALS", NumArgs: 2, Precedence: 40, Handler: notEqualsOperator}
+
+//createmap needs to be above union, as we use union to build the components of the objects
+var createMapOpType = &operationType{Type: "CREATE_MAP", NumArgs: 2, Precedence: 15, Handler: createMapOperator}
 
 var shortPipeOpType = &operationType{Type: "SHORT_PIPE", NumArgs: 2, Precedence: 45, Handler: pipeOperator}
 
 var lengthOpType = &operationType{Type: "LENGTH", NumArgs: 0, Precedence: 50, Handler: lengthOperator}
 var collectOpType = &operationType{Type: "COLLECT", NumArgs: 0, Precedence: 50, Handler: collectOperator}
+
+var anyOpType = &operationType{Type: "ANY", NumArgs: 0, Precedence: 50, Handler: anyOperator}
+var allOpType = &operationType{Type: "ALL", NumArgs: 0, Precedence: 50, Handler: allOperator}
+var anyConditionOpType = &operationType{Type: "ANY_CONDITION", NumArgs: 1, Precedence: 50, Handler: anyOperator}
+var allConditionOpType = &operationType{Type: "ALL_CONDITION", NumArgs: 1, Precedence: 50, Handler: allOperator}
+
+var toEntriesOpType = &operationType{Type: "TO_ENTRIES", NumArgs: 0, Precedence: 50, Handler: toEntriesOperator}
+var fromEntriesOpType = &operationType{Type: "FROM_ENTRIES", NumArgs: 0, Precedence: 50, Handler: fromEntriesOperator}
+var withEntriesOpType = &operationType{Type: "WITH_ENTRIES", NumArgs: 1, Precedence: 50, Handler: withEntriesOperator}
+
 var splitDocumentOpType = &operationType{Type: "SPLIT_DOC", NumArgs: 0, Precedence: 50, Handler: splitDocumentOperator}
+var getVariableOpType = &operationType{Type: "GET_VARIABLE", NumArgs: 0, Precedence: 55, Handler: getVariableOperator}
 var getStyleOpType = &operationType{Type: "GET_STYLE", NumArgs: 0, Precedence: 50, Handler: getStyleOperator}
 var getTagOpType = &operationType{Type: "GET_TAG", NumArgs: 0, Precedence: 50, Handler: getTagOperator}
 var getCommentOpType = &operationType{Type: "GET_COMMENT", NumArgs: 0, Precedence: 50, Handler: getCommentsOperator}
@@ -65,26 +82,28 @@ var getPathOpType = &operationType{Type: "GET_PATH", NumArgs: 0, Precedence: 50,
 var explodeOpType = &operationType{Type: "EXPLODE", NumArgs: 1, Precedence: 50, Handler: explodeOperator}
 var sortKeysOpType = &operationType{Type: "SORT_KEYS", NumArgs: 1, Precedence: 50, Handler: sortKeysOperator}
 var joinStringOpType = &operationType{Type: "JOIN", NumArgs: 1, Precedence: 50, Handler: joinStringOperator}
+var subStringOpType = &operationType{Type: "SUBSTR", NumArgs: 1, Precedence: 50, Handler: substituteStringOperator}
 var splitStringOpType = &operationType{Type: "SPLIT", NumArgs: 1, Precedence: 50, Handler: splitStringOperator}
 
 var keysOpType = &operationType{Type: "KEYS", NumArgs: 0, Precedence: 50, Handler: keysOperator}
 
 var collectObjectOpType = &operationType{Type: "COLLECT_OBJECT", NumArgs: 0, Precedence: 50, Handler: collectObjectOperator}
-var traversePathOpType = &operationType{Type: "TRAVERSE_PATH", NumArgs: 0, Precedence: 50, Handler: traversePathOperator}
-var traverseArrayOpType = &operationType{Type: "TRAVERSE_ARRAY", NumArgs: 1, Precedence: 50, Handler: traverseArrayOperator}
+var traversePathOpType = &operationType{Type: "TRAVERSE_PATH", NumArgs: 0, Precedence: 55, Handler: traversePathOperator}
+var traverseArrayOpType = &operationType{Type: "TRAVERSE_ARRAY", NumArgs: 2, Precedence: 50, Handler: traverseArrayOperator}
 
-var selfReferenceOpType = &operationType{Type: "SELF", NumArgs: 0, Precedence: 50, Handler: selfOperator}
+var selfReferenceOpType = &operationType{Type: "SELF", NumArgs: 0, Precedence: 55, Handler: selfOperator}
 var valueOpType = &operationType{Type: "VALUE", NumArgs: 0, Precedence: 50, Handler: valueOperator}
 var envOpType = &operationType{Type: "ENV", NumArgs: 0, Precedence: 50, Handler: envOperator}
 var notOpType = &operationType{Type: "NOT", NumArgs: 0, Precedence: 50, Handler: notOperator}
-var emptyOpType = &operationType{Type: "EMPTY", NumArgs: 50, Handler: emptyOperator}
+var emptyOpType = &operationType{Type: "EMPTY", Precedence: 50, Handler: emptyOperator}
 
 var recursiveDescentOpType = &operationType{Type: "RECURSIVE_DESCENT", NumArgs: 0, Precedence: 50, Handler: recursiveDescentOperator}
 
 var selectOpType = &operationType{Type: "SELECT", NumArgs: 1, Precedence: 50, Handler: selectOperator}
 var hasOpType = &operationType{Type: "HAS", NumArgs: 1, Precedence: 50, Handler: hasOperator}
+var uniqueOpType = &operationType{Type: "UNIQUE", NumArgs: 0, Precedence: 50, Handler: unique}
+var uniqueByOpType = &operationType{Type: "UNIQUE_BY", NumArgs: 1, Precedence: 50, Handler: uniqueBy}
 var deleteChildOpType = &operationType{Type: "DELETE", NumArgs: 1, Precedence: 40, Handler: deleteChildOperator}
-var deleteImmediateChildOpType = &operationType{Type: "DELETE_IMMEDIATE_CHILD", NumArgs: 1, Precedence: 40, Handler: deleteImmediateChildOperator}
 
 type Operation struct {
 	OperationType *operationType

pkg/yqlib/operator_add.go

@@ -3,7 +3,6 @@ package yqlib
 import (
 	"fmt"
 
-	"container/list"
 	"strconv"
 
 	yaml "gopkg.in/yaml.v3"
@@ -15,12 +14,12 @@ func createAddOp(lhs *ExpressionNode, rhs *ExpressionNode) *ExpressionNode {
 		Rhs: rhs}
 }
 
-func addAssignOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func addAssignOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	assignmentOp := &Operation{OperationType: assignOpType}
-	assignmentOp.UpdateAssign = false
-
-	assignmentOpNode := &ExpressionNode{Operation: assignmentOp, Lhs: expressionNode.Lhs, Rhs: createAddOp(expressionNode.Lhs, expressionNode.Rhs)}
-	return d.GetMatchingNodes(matchingNodes, assignmentOpNode)
+	assignmentOp.UpdateAssign = true
+	selfExpression := &ExpressionNode{Operation: &Operation{OperationType: selfReferenceOpType}}
+	assignmentOpNode := &ExpressionNode{Operation: assignmentOp, Lhs: expressionNode.Lhs, Rhs: createAddOp(selfExpression, expressionNode.Rhs)}
+	return d.GetMatchingNodes(context, assignmentOpNode)
 }
 
 func toNodes(candidate *CandidateNode) []*yaml.Node {
@@ -37,13 +36,13 @@ func toNodes(candidate *CandidateNode) []*yaml.Node {
 
 }
 
-func addOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func addOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("Add operator")
 
-	return crossFunction(d, matchingNodes, expressionNode, add)
+	return crossFunction(d, context.ReadOnlyClone(), expressionNode, add, false)
 }
 
-func add(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+func add(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
 	lhs.Node = unwrapDoc(lhs.Node)
 	rhs.Node = unwrapDoc(rhs.Node)
 

pkg/yqlib/operator_add_test.go

@@ -14,6 +14,22 @@ var addOperatorScenarios = []expressionScenario{
 			"D0, P[1 a], (!!int)::3\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `{}`,
+		expression: "(.a + .b) as $x",
+		expected: []string{
+			"D0, P[], (doc)::{}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: 0`,
+		expression: ".a += .b.c",
+		expected: []string{
+			"D0, P[], (doc)::a: 0\n",
+		},
+	},
 	{
 		description: "Concatenate and assign arrays",
 		document:    `{a: {val: thing, b: [cat,dog]}}`,
@@ -63,7 +79,7 @@ var addOperatorScenarios = []expressionScenario{
 		},
 	},
 	{
-		description: "Update array (append)",
+		description: "Append to array",
 		document:    `{a: [1,2], b: [3,4]}`,
 		expression:  `.a = .a + .b`,
 		expected: []string{
@@ -71,17 +87,17 @@ var addOperatorScenarios = []expressionScenario{
 		},
 	},
 	{
-		description: "String concatenation",
-		document:    `{a: cat, b: meow}`,
-		expression:  `.a = .a + .b`,
+		description: "Relative append",
+		document:    `a: { a1: {b: [cat]}, a2: {b: [dog]}, a3: {} }`,
+		expression:  `.a[].b += ["mouse"]`,
 		expected: []string{
-			"D0, P[], (doc)::{a: catmeow, b: meow}\n",
+			"D0, P[], (doc)::a: {a1: {b: [cat, mouse]}, a2: {b: [dog, mouse]}, a3: {b: [mouse]}}\n",
 		},
 	},
 	{
-		description: "Relative string concatenation",
+		description: "String concatenation",
 		document:    `{a: cat, b: meow}`,
-		expression:  `.a += .b`,
+		expression:  `.a = .a + .b`,
 		expected: []string{
 			"D0, P[], (doc)::{a: catmeow, b: meow}\n",
 		},
@@ -105,11 +121,11 @@ var addOperatorScenarios = []expressionScenario{
 		},
 	},
 	{
-		description: "Increment number",
-		document:    `{a: 3}`,
-		expression:  `.a += 1`,
+		description: "Increment numbers",
+		document:    `{a: 3, b: 5}`,
+		expression:  `.[] += 1`,
 		expected: []string{
-			"D0, P[], (doc)::{a: 4}\n",
+			"D0, P[], (doc)::{a: 4, b: 6}\n",
 		},
 	},
 	{

pkg/yqlib/operator_alternative.go

@@ -1,18 +1,14 @@
 package yqlib
 
-import (
-	"container/list"
-)
-
-// corssFunction no matches
-// can boolean use crossfunction
-
-func alternativeOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func alternativeOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- alternative")
-	return crossFunction(d, matchingNodes, expressionNode, alternativeFunc)
+	return crossFunction(d, context.ReadOnlyClone(), expressionNode, alternativeFunc, true)
 }
 
-func alternativeFunc(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+func alternativeFunc(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+	if lhs == nil {
+		return rhs, nil
+	}
 	lhs.Node = unwrapDoc(lhs.Node)
 	rhs.Node = unwrapDoc(rhs.Node)
 	log.Debugf("Alternative LHS: %v", lhs.Node.Tag)

pkg/yqlib/operator_alternative_test.go

@@ -5,6 +5,20 @@ import (
 )
 
 var alternativeOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:    true,
+		expression: `.b // .c`,
+		document:   `a: bridge`,
+		expected:   []string{},
+	},
+	{
+		skipDoc:    true,
+		expression: `(.b // "hello") as $x`,
+		document:   `a: bridge`,
+		expected: []string{
+			"D0, P[], (doc)::a: bridge\n",
+		},
+	},
 	{
 		description: "LHS is defined",
 		expression:  `.a // "hello"`,
@@ -13,6 +27,14 @@ var alternativeOperatorScenarios = []expressionScenario{
 			"D0, P[a], (!!str)::bridge\n",
 		},
 	},
+	{
+		expression: `select(tag == "seq") // "cat"`,
+		skipDoc:    true,
+		document:   `a: frog`,
+		expected: []string{
+			"D0, P[], (!!str)::cat\n",
+		},
+	},
 	{
 		description: "LHS is not defined",
 		expression:  `.a // "hello"`,

pkg/yqlib/operator_anchors_aliases.go

@@ -6,146 +6,148 @@ import (
 	yaml "gopkg.in/yaml.v3"
 )
 
-func assignAliasOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func assignAliasOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
 	log.Debugf("AssignAlias operator!")
 
 	aliasName := ""
 	if !expressionNode.Operation.UpdateAssign {
-		rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
-		if rhs.Front() != nil {
-			aliasName = rhs.Front().Value.(*CandidateNode).Node.Value
+		if rhs.MatchingNodes.Front() != nil {
+			aliasName = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 		}
 	}
 
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
-	for el := lhs.Front(); el != nil; el = el.Next() {
+	for el := lhs.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		log.Debugf("Setting aliasName : %v", candidate.GetKey())
 
 		if expressionNode.Operation.UpdateAssign {
-			rhs, err := d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+			rhs, err := d.GetMatchingNodes(context.SingleReadonlyChildContext(candidate), expressionNode.Rhs)
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
-			if rhs.Front() != nil {
-				aliasName = rhs.Front().Value.(*CandidateNode).Node.Value
+			if rhs.MatchingNodes.Front() != nil {
+				aliasName = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 			}
 		}
 
-		candidate.Node.Kind = yaml.AliasNode
-		candidate.Node.Value = aliasName
+		if aliasName != "" {
+			candidate.Node.Kind = yaml.AliasNode
+			candidate.Node.Value = aliasName
+		}
 	}
-	return matchingNodes, nil
+	return context, nil
 }
 
-func getAliasOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getAliasOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("GetAlias operator!")
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := &yaml.Node{Kind: yaml.ScalarNode, Value: candidate.Node.Value, Tag: "!!str"}
 		result := candidate.CreateChild(nil, node)
 		results.PushBack(result)
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }
 
-func assignAnchorOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func assignAnchorOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
 	log.Debugf("AssignAnchor operator!")
 
 	anchorName := ""
 	if !expressionNode.Operation.UpdateAssign {
-		rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
-		if rhs.Front() != nil {
-			anchorName = rhs.Front().Value.(*CandidateNode).Node.Value
+		if rhs.MatchingNodes.Front() != nil {
+			anchorName = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 		}
 	}
 
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
-	for el := lhs.Front(); el != nil; el = el.Next() {
+	for el := lhs.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		log.Debugf("Setting anchorName of : %v", candidate.GetKey())
 
 		if expressionNode.Operation.UpdateAssign {
-			rhs, err := d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+			rhs, err := d.GetMatchingNodes(context.SingleReadonlyChildContext(candidate), expressionNode.Rhs)
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 
-			if rhs.Front() != nil {
-				anchorName = rhs.Front().Value.(*CandidateNode).Node.Value
+			if rhs.MatchingNodes.Front() != nil {
+				anchorName = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 			}
 		}
 
 		candidate.Node.Anchor = anchorName
 	}
-	return matchingNodes, nil
+	return context, nil
 }
 
-func getAnchorOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getAnchorOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("GetAnchor operator!")
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		anchor := candidate.Node.Anchor
 		node := &yaml.Node{Kind: yaml.ScalarNode, Value: anchor, Tag: "!!str"}
 		result := candidate.CreateChild(nil, node)
 		results.PushBack(result)
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }
 
-func explodeOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func explodeOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- ExplodeOperation")
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 
-		rhs, err := d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context.SingleChildContext(candidate), expressionNode.Rhs)
 
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
-		for childEl := rhs.Front(); childEl != nil; childEl = childEl.Next() {
-			err = explodeNode(childEl.Value.(*CandidateNode).Node)
+		for childEl := rhs.MatchingNodes.Front(); childEl != nil; childEl = childEl.Next() {
+			err = explodeNode(childEl.Value.(*CandidateNode).Node, context)
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 		}
 
 	}
 
-	return matchMap, nil
+	return context, nil
 }
 
-func explodeNode(node *yaml.Node) error {
+func explodeNode(node *yaml.Node, context Context) error {
 	node.Anchor = ""
 	switch node.Kind {
 	case yaml.SequenceNode, yaml.DocumentNode:
 		for index, contentNode := range node.Content {
 			log.Debugf("exploding index %v", index)
-			errorInContent := explodeNode(contentNode)
+			errorInContent := explodeNode(contentNode, context)
 			if errorInContent != nil {
 				return errorInContent
 			}
@@ -169,23 +171,23 @@ func explodeNode(node *yaml.Node) error {
 			valueNode := node.Content[index+1]
 			log.Debugf("traversing %v", keyNode.Value)
 			if keyNode.Value != "<<" {
-				err := overrideEntry(node, keyNode, valueNode, index, newContent)
+				err := overrideEntry(node, keyNode, valueNode, index, context.ChildContext(newContent))
 				if err != nil {
 					return err
 				}
 			} else {
 				if valueNode.Kind == yaml.SequenceNode {
 					log.Debugf("an alias merge list!")
-					for index := 0; index < len(valueNode.Content); index = index + 1 {
+					for index := len(valueNode.Content) - 1; index >= 0; index = index - 1 {
 						aliasNode := valueNode.Content[index]
-						err := applyAlias(node, aliasNode.Alias, index, newContent)
+						err := applyAlias(node, aliasNode.Alias, index, context.ChildContext(newContent))
 						if err != nil {
 							return err
 						}
 					}
 				} else {
 					log.Debugf("an alias merge!")
-					err := applyAlias(node, valueNode.Alias, index, newContent)
+					err := applyAlias(node, valueNode.Alias, index, context.ChildContext(newContent))
 					if err != nil {
 						return err
 					}
@@ -205,7 +207,7 @@ func explodeNode(node *yaml.Node) error {
 	}
 }
 
-func applyAlias(node *yaml.Node, alias *yaml.Node, aliasIndex int, newContent *list.List) error {
+func applyAlias(node *yaml.Node, alias *yaml.Node, aliasIndex int, newContent Context) error {
 	if alias == nil {
 		return nil
 	}
@@ -221,15 +223,15 @@ func applyAlias(node *yaml.Node, alias *yaml.Node, aliasIndex int, newContent *l
 	return nil
 }
 
-func overrideEntry(node *yaml.Node, key *yaml.Node, value *yaml.Node, startIndex int, newContent *list.List) error {
+func overrideEntry(node *yaml.Node, key *yaml.Node, value *yaml.Node, startIndex int, newContent Context) error {
 
-	err := explodeNode(value)
+	err := explodeNode(value, newContent)
 
 	if err != nil {
 		return err
 	}
 
-	for newEl := newContent.Front(); newEl != nil; newEl = newEl.Next() {
+	for newEl := newContent.MatchingNodes.Front(); newEl != nil; newEl = newEl.Next() {
 		valueEl := newEl.Next() // move forward twice
 		keyNode := newEl.Value.(*yaml.Node)
 		log.Debugf("checking new content %v:%v", keyNode.Value, valueEl.Value.(*yaml.Node).Value)
@@ -250,12 +252,12 @@ func overrideEntry(node *yaml.Node, key *yaml.Node, value *yaml.Node, startIndex
 		}
 	}
 
-	err = explodeNode(key)
+	err = explodeNode(key, newContent)
 	if err != nil {
 		return err
 	}
 	log.Debugf("adding %v:%v", key.Value, value.Value)
-	newContent.PushBack(key)
-	newContent.PushBack(value)
+	newContent.MatchingNodes.PushBack(key)
+	newContent.MatchingNodes.PushBack(value)
 	return nil
 }

pkg/yqlib/operator_anchors_aliases_test.go

@@ -4,7 +4,36 @@ import (
 	"testing"
 )
 
+var specDocument = `- &CENTER { x: 1, y: 2 }
+- &LEFT { x: 0, y: 2 }
+- &BIG { r: 10 }
+- &SMALL { r: 1 }
+`
+
+var expectedSpecResult = "D0, P[4], (!!map)::x: 1\ny: 2\nr: 10\n"
+
 var anchorOperatorScenarios = []expressionScenario{
+	{
+		description:    "Merge one map",
+		subdescription: "see https://yaml.org/type/merge.html",
+		document:       specDocument + "- << : *CENTER\n  r: 10\n",
+		expression:     ".[4] | explode(.)",
+		expected:       []string{expectedSpecResult},
+	},
+	{
+		description:    "Merge multiple maps",
+		subdescription: "see https://yaml.org/type/merge.html",
+		document:       specDocument + "- << : [ *CENTER, *BIG ]\n",
+		expression:     ".[4] | explode(.)",
+		expected:       []string{"D0, P[4], (!!map)::r: 10\nx: 1\ny: 2\n"},
+	},
+	{
+		description:    "Override",
+		subdescription: "see https://yaml.org/type/merge.html",
+		document:       specDocument + "- << : [ *BIG, *LEFT, *SMALL ]\n  x: 1\n",
+		expression:     ".[4] | explode(.)",
+		expected:       []string{"D0, P[4], (!!map)::r: 10\nx: 1\ny: 2\n"},
+	},
 	{
 		description: "Get anchor",
 		document:    `a: &billyBob cat`,
@@ -29,6 +58,22 @@ var anchorOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::a: &cat {b: cat}\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `a: {c: cat}`,
+		expression: `.a anchor |= .b`,
+		expected: []string{
+			"D0, P[], (doc)::a: {c: cat}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: {c: cat}`,
+		expression: `.a anchor = .b`,
+		expected: []string{
+			"D0, P[], (doc)::a: {c: cat}\n",
+		},
+	},
 	{
 		description: "Get alias",
 		document:    `{b: &billyBob meow, a: *billyBob}`,
@@ -45,6 +90,30 @@ var anchorOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::{b: &meow purr, a: *meow}\n",
 		},
 	},
+	{
+		description: "Set alias to blank does nothing",
+		document:    `{b: &meow purr, a: cat}`,
+		expression:  `.a alias = ""`,
+		expected: []string{
+			"D0, P[], (doc)::{b: &meow purr, a: cat}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{b: &meow purr, a: cat}`,
+		expression: `.a alias = .c`,
+		expected: []string{
+			"D0, P[], (doc)::{b: &meow purr, a: cat}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{b: &meow purr, a: cat}`,
+		expression: `.a alias |= .c`,
+		expected: []string{
+			"D0, P[], (doc)::{b: &meow purr, a: cat}\n",
+		},
+	},
 	{
 		description: "Set alias relatively using assign-update",
 		document:    `{b: &meow purr, a: {f: meow}}`,
@@ -91,9 +160,9 @@ bar:
     c: bar_c
 foobarList:
     b: bar_b
-    a: foo_a
-    thing: bar_thing
+    thing: foo_thing
     c: foobarList_c
+    a: foo_a
 foobar:
     c: foo_c
     a: foo_a
@@ -106,7 +175,7 @@ foobar:
 		expression: `.foo* | explode(.) | (. style="flow")`,
 		expected: []string{
 			"D0, P[foo], (!!map)::{a: foo_a, thing: foo_thing, c: foo_c}\n",
-			"D0, P[foobarList], (!!map)::{b: bar_b, a: foo_a, thing: bar_thing, c: foobarList_c}\n",
+			"D0, P[foobarList], (!!map)::{b: bar_b, thing: foo_thing, c: foobarList_c, a: foo_a}\n",
 			"D0, P[foobar], (!!map)::{c: foo_c, a: foo_a, thing: foobar_thing}\n",
 		},
 	},
@@ -116,7 +185,7 @@ foobar:
 		expression: `.foo* | explode(explode(.)) | (. style="flow")`,
 		expected: []string{
 			"D0, P[foo], (!!map)::{a: foo_a, thing: foo_thing, c: foo_c}\n",
-			"D0, P[foobarList], (!!map)::{b: bar_b, a: foo_a, thing: bar_thing, c: foobarList_c}\n",
+			"D0, P[foobarList], (!!map)::{b: bar_b, thing: foo_thing, c: foobarList_c, a: foo_a}\n",
 			"D0, P[foobar], (!!map)::{c: foo_c, a: foo_a, thing: foobar_thing}\n",
 		},
 	},
@@ -130,7 +199,7 @@ foobar:
 	},
 }
 
-func TestAnchorAliaseOperatorScenarios(t *testing.T) {
+func TestAnchorAliasOperatorScenarios(t *testing.T) {
 	for _, tt := range anchorOperatorScenarios {
 		testScenario(t, &tt)
 	}

pkg/yqlib/operator_assign.go

@@ -1,30 +1,28 @@
 package yqlib
 
-import "container/list"
-
-func assignUpdateOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
+func assignUpdateOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-	var rhs *list.List
+	var rhs Context
 	if !expressionNode.Operation.UpdateAssign {
-		rhs, err = d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+		rhs, err = d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
 	}
 
-	for el := lhs.Front(); el != nil; el = el.Next() {
+	for el := lhs.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 
 		if expressionNode.Operation.UpdateAssign {
-			rhs, err = d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+			rhs, err = d.GetMatchingNodes(context.SingleChildContext(candidate), expressionNode.Rhs)
 		}
 
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
 		// grab the first value
-		first := rhs.Front()
+		first := rhs.MatchingNodes.Front()
 
 		if first != nil {
 			rhsCandidate := first.Value.(*CandidateNode)
@@ -33,30 +31,31 @@ func assignUpdateOperator(d *dataTreeNavigator, matchingNodes *list.List, expres
 		}
 	}
 
-	return matchingNodes, nil
+	return context, nil
 }
 
 // does not update content or values
-func assignAttributesOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
+func assignAttributesOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	log.Debug("getting lhs matching nodes for update")
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-	for el := lhs.Front(); el != nil; el = el.Next() {
+	for el := lhs.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 
-		rhs, err := d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context.SingleReadonlyChildContext(candidate), expressionNode.Rhs)
 
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
 		// grab the first value
-		first := rhs.Front()
+		first := rhs.MatchingNodes.Front()
 
 		if first != nil {
 			candidate.UpdateAttributesFrom(first.Value.(*CandidateNode))
 		}
 	}
-	return matchingNodes, nil
+	return context, nil
 }

pkg/yqlib/operator_assign_test.go

@@ -12,6 +12,22 @@ var assignOperatorScenarios = []expressionScenario{
 			"D0, P[], ()::a:\n    b: cat\nx: frog\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   "{}",
+		expression: `.a |= .b`,
+		expected: []string{
+			"D0, P[], (doc)::{a: null}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "{}",
+		expression: `.a = .b`,
+		expected: []string{
+			"D0, P[], (doc)::{a: null}\n",
+		},
+	},
 	{
 		description: "Update node to be the child value",
 		document:    `{a: {b: {g: foof}}}`,
@@ -143,5 +159,5 @@ func TestAssignOperatorScenarios(t *testing.T) {
 	for _, tt := range assignOperatorScenarios {
 		testScenario(t, &tt)
 	}
-	documentScenarios(t, "Assign", assignOperatorScenarios)
+	documentScenarios(t, "Assign (Update)", assignOperatorScenarios)
 }

pkg/yqlib/operator_booleans.go

@@ -2,14 +2,13 @@ package yqlib
 
 import (
 	"container/list"
+	"fmt"
 
 	yaml "gopkg.in/yaml.v3"
 )
 
-func isTruthy(c *CandidateNode) (bool, error) {
-	node := unwrapDoc(c.Node)
+func isTruthyNode(node *yaml.Node) (bool, error) {
 	value := true
-
 	if node.Tag == "!!null" {
 		return false, nil
 	}
@@ -23,56 +22,146 @@ func isTruthy(c *CandidateNode) (bool, error) {
 	return value, nil
 }
 
+func isTruthy(c *CandidateNode) (bool, error) {
+	node := unwrapDoc(c.Node)
+	return isTruthyNode(node)
+}
+
 type boolOp func(bool, bool) bool
 
-func performBoolOp(op boolOp) func(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
-	return func(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
-		lhs.Node = unwrapDoc(lhs.Node)
-		rhs.Node = unwrapDoc(rhs.Node)
+func performBoolOp(op boolOp) func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+	return func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+		owner := lhs
 
-		lhsTrue, errDecoding := isTruthy(lhs)
-		if errDecoding != nil {
-			return nil, errDecoding
+		if lhs == nil && rhs == nil {
+			owner = &CandidateNode{}
+		} else if lhs == nil {
+			owner = rhs
 		}
 
-		rhsTrue, errDecoding := isTruthy(rhs)
-		if errDecoding != nil {
-			return nil, errDecoding
-		}
+		var errDecoding error
+		lhsTrue := false
+		if lhs != nil {
+			lhs.Node = unwrapDoc(lhs.Node)
+			lhsTrue, errDecoding = isTruthy(lhs)
 
-		return createBooleanCandidate(lhs, op(lhsTrue, rhsTrue)), nil
+			if errDecoding != nil {
+				return nil, errDecoding
+			}
+		}
+		log.Debugf("-- lhsTrue", lhsTrue)
+
+		rhsTrue := false
+		if rhs != nil {
+			rhs.Node = unwrapDoc(rhs.Node)
+			rhsTrue, errDecoding = isTruthy(rhs)
+			if errDecoding != nil {
+				return nil, errDecoding
+			}
+		}
+		log.Debugf("-- rhsTrue", rhsTrue)
+
+		return createBooleanCandidate(owner, op(lhsTrue, rhsTrue)), nil
 	}
 }
 
-func orOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	log.Debugf("-- orOp")
-	return crossFunction(d, matchingNodes, expressionNode, performBoolOp(
-		func(b1 bool, b2 bool) bool {
-			return b1 || b2
-		}))
+func findBoolean(wantBool bool, d *dataTreeNavigator, context Context, expressionNode *ExpressionNode, sequenceNode *yaml.Node) (bool, error) {
+	for _, node := range sequenceNode.Content {
+
+		if expressionNode != nil {
+			//need to evaluate the expression against the node
+			candidate := &CandidateNode{Node: node}
+			rhs, err := d.GetMatchingNodes(context.SingleReadonlyChildContext(candidate), expressionNode)
+			if err != nil {
+				return false, err
+			}
+			if rhs.MatchingNodes.Len() > 0 {
+				node = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node
+			} else {
+				// no results found, ignore this entry
+				continue
+			}
+		}
+
+		truthy, err := isTruthyNode(node)
+		if err != nil {
+			return false, err
+		}
+		if truthy == wantBool {
+			return true, nil
+		}
+	}
+	return false, nil
 }
 
-func andOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func allOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	var results = list.New()
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+		candidateNode := unwrapDoc(candidate.Node)
+		if candidateNode.Kind != yaml.SequenceNode {
+			return Context{}, fmt.Errorf("any only supports arrays, was %v", candidateNode.Tag)
+		}
+		booleanResult, err := findBoolean(false, d, context, expressionNode.Rhs, candidateNode)
+		if err != nil {
+			return Context{}, err
+		}
+		result := createBooleanCandidate(candidate, !booleanResult)
+		results.PushBack(result)
+	}
+	return context.ChildContext(results), nil
+}
+
+func anyOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	var results = list.New()
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+		candidateNode := unwrapDoc(candidate.Node)
+		if candidateNode.Kind != yaml.SequenceNode {
+			return Context{}, fmt.Errorf("any only supports arrays, was %v", candidateNode.Tag)
+		}
+		booleanResult, err := findBoolean(true, d, context, expressionNode.Rhs, candidateNode)
+		if err != nil {
+			return Context{}, err
+		}
+		result := createBooleanCandidate(candidate, booleanResult)
+		results.PushBack(result)
+	}
+	return context.ChildContext(results), nil
+}
+
+func orOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	log.Debugf("-- orOp")
+	return crossFunction(d, context.ReadOnlyClone(), expressionNode, performBoolOp(
+		func(b1 bool, b2 bool) bool {
+			log.Debugf("-- peformingOrOp with %v and %v", b1, b2)
+			return b1 || b2
+		}), true)
+}
+
+func andOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- AndOp")
-	return crossFunction(d, matchingNodes, expressionNode, performBoolOp(
+	return crossFunction(d, context.ReadOnlyClone(), expressionNode, performBoolOp(
 		func(b1 bool, b2 bool) bool {
 			return b1 && b2
-		}))
+		}), true)
 }
 
-func notOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func notOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- notOperation")
 	var results = list.New()
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		log.Debug("notOperation checking %v", candidate)
 		truthy, errDecoding := isTruthy(candidate)
 		if errDecoding != nil {
-			return nil, errDecoding
+			return Context{}, errDecoding
 		}
 		result := createBooleanCandidate(candidate, !truthy)
 		results.PushBack(result)
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_booleans_test.go

@@ -6,14 +6,38 @@ import (
 
 var booleanOperatorScenarios = []expressionScenario{
 	{
-		description: "OR example",
+		description: "`or` example",
 		expression:  `true or false`,
 		expected: []string{
 			"D0, P[], (!!bool)::true\n",
 		},
 	},
 	{
-		description: "AND example",
+		skipDoc:    true,
+		document:   "b: hi",
+		expression: `.a or .c`,
+		expected: []string{
+			"D0, P[], (!!bool)::false\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "b: hi",
+		expression: `select(.a or .b)`,
+		expected: []string{
+			"D0, P[], (doc)::b: hi\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "b: hi",
+		expression: `select((.a and .b) | not)`,
+		expected: []string{
+			"D0, P[], (doc)::b: hi\n",
+		},
+	},
+	{
+		description: "`and` example",
 		expression:  `true and false`,
 		expected: []string{
 			"D0, P[], (!!bool)::false\n",
@@ -27,6 +51,86 @@ var booleanOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!seq)::- {a: bird, b: dog}\n- {a: cat, b: fly}\n",
 		},
 	},
+	{
+		description: "`any` returns true if any boolean in a given array is true",
+		document:    `[false, true]`,
+		expression:  "any",
+		expected: []string{
+			"D0, P[], (!!bool)::true\n",
+		},
+	},
+	{
+		description: "`any` returns false for an empty array",
+		document:    `[]`,
+		expression:  "any",
+		expected: []string{
+			"D0, P[], (!!bool)::false\n",
+		},
+	},
+	{
+		description: "`any_c` returns true if any element in the array is true for the given condition.",
+		document:    "a: [rad, awesome]\nb: [meh, whatever]",
+		expression:  `.[] |= any_c(. == "awesome")`,
+		expected: []string{
+			"D0, P[], (doc)::a: true\nb: false\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[{pet: cat}]`,
+		expression: `any_c(.name == "harry") as $c`,
+		expected: []string{
+			"D0, P[], (doc)::[{pet: cat}]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[{pet: cat}]`,
+		expression: `all_c(.name == "harry") as $c`,
+		expected: []string{
+			"D0, P[], (doc)::[{pet: cat}]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[false, false]`,
+		expression: "any",
+		expected: []string{
+			"D0, P[], (!!bool)::false\n",
+		},
+	},
+	{
+		description: "`all` returns true if all booleans in a given array are true",
+		document:    `[true, true]`,
+		expression:  "all",
+		expected: []string{
+			"D0, P[], (!!bool)::true\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[false, true]`,
+		expression: "all",
+		expected: []string{
+			"D0, P[], (!!bool)::false\n",
+		},
+	},
+	{
+		description: "`all` returns true for an empty array",
+		document:    `[]`,
+		expression:  "all",
+		expected: []string{
+			"D0, P[], (!!bool)::true\n",
+		},
+	},
+	{
+		description: "`all_c` returns true if all elements in the array are true for the given condition.",
+		document:    "a: [rad, awesome]\nb: [meh, 12]",
+		expression:  `.[] |= all_c(tag == "!!str")`,
+		expected: []string{
+			"D0, P[], (doc)::a: true\nb: false\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		expression: `false or false`,
@@ -45,6 +149,22 @@ var booleanOperatorScenarios = []expressionScenario{
 			"D0, P[b], (!!bool)::true\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `{}`,
+		expression: `(.a.b or .c) as $x`,
+		expected: []string{
+			"D0, P[], (doc)::{}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{}`,
+		expression: `(.a.b and .c) as $x`,
+		expected: []string{
+			"D0, P[], (doc)::{}\n",
+		},
+	},
 	{
 		description: "Not true is false",
 		expression:  `true | not`,

pkg/yqlib/operator_collect.go

@@ -6,21 +6,21 @@ import (
 	yaml "gopkg.in/yaml.v3"
 )
 
-func collectOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func collectOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- collectOperation")
 
-	if matchMap.Len() == 0 {
+	if context.MatchingNodes.Len() == 0 {
 		node := &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq", Value: "[]"}
 		candidate := &CandidateNode{Node: node}
-		return nodeToMap(candidate), nil
+		return context.SingleChildContext(candidate), nil
 	}
 
 	var results = list.New()
 
 	node := &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq"}
 	var collectC *CandidateNode
-	if matchMap.Front() != nil {
-		collectC = matchMap.Front().Value.(*CandidateNode).CreateChild(nil, node)
+	if context.MatchingNodes.Front() != nil {
+		collectC = context.MatchingNodes.Front().Value.(*CandidateNode).CreateChild(nil, node)
 		if len(collectC.Path) > 0 {
 			collectC.Path = collectC.Path[:len(collectC.Path)-1]
 		}
@@ -28,7 +28,7 @@ func collectOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *
 		collectC = &CandidateNode{Node: node}
 	}
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		log.Debugf("Collecting %v", NodeToString(candidate))
 		node.Content = append(node.Content, unwrapDoc(candidate.Node))
@@ -36,5 +36,5 @@ func collectOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *
 
 	results.PushBack(collectC)
 
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_collect_object.go

@@ -17,22 +17,25 @@ import (
 ...
 */
 
-func collectObjectOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func collectObjectOperator(d *dataTreeNavigator, originalContext Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- collectObjectOperation")
 
-	if matchMap.Len() == 0 {
+	context := originalContext.Clone()
+	context.DontAutoCreate = false
+
+	if context.MatchingNodes.Len() == 0 {
 		node := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map", Value: "{}"}
 		candidate := &CandidateNode{Node: node}
-		return nodeToMap(candidate), nil
+		return context.SingleChildContext(candidate), nil
 	}
-	first := matchMap.Front().Value.(*CandidateNode)
+	first := context.MatchingNodes.Front().Value.(*CandidateNode)
 	var rotated []*list.List = make([]*list.List, len(first.Node.Content))
 
 	for i := 0; i < len(first.Node.Content); i++ {
 		rotated[i] = list.New()
 	}
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidateNode := el.Value.(*CandidateNode)
 		for i := 0; i < len(first.Node.Content); i++ {
 			rotated[i].PushBack(candidateNode.CreateChild(i, candidateNode.Node.Content[i]))
@@ -41,59 +44,59 @@ func collectObjectOperator(d *dataTreeNavigator, matchMap *list.List, expression
 
 	newObject := list.New()
 	for i := 0; i < len(first.Node.Content); i++ {
-		additions, err := collect(d, list.New(), rotated[i])
+		additions, err := collect(d, context.ChildContext(list.New()), rotated[i])
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
-		newObject.PushBackList(additions)
+		newObject.PushBackList(additions.MatchingNodes)
 	}
 
-	return newObject, nil
+	return context.ChildContext(newObject), nil
 
 }
 
-func collect(d *dataTreeNavigator, aggregate *list.List, remainingMatches *list.List) (*list.List, error) {
+func collect(d *dataTreeNavigator, context Context, remainingMatches *list.List) (Context, error) {
 	if remainingMatches.Len() == 0 {
-		return aggregate, nil
+		return context, nil
 	}
 
 	candidate := remainingMatches.Remove(remainingMatches.Front()).(*CandidateNode)
 
-	splatted, err := splat(d, nodeToMap(candidate),
+	splatted, err := splat(d, context.SingleChildContext(candidate),
 		traversePreferences{DontFollowAlias: true, IncludeMapKeys: false})
 
-	for splatEl := splatted.Front(); splatEl != nil; splatEl = splatEl.Next() {
+	for splatEl := splatted.MatchingNodes.Front(); splatEl != nil; splatEl = splatEl.Next() {
 		splatEl.Value.(*CandidateNode).Path = nil
 	}
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
-	if aggregate.Len() == 0 {
+	if context.MatchingNodes.Len() == 0 {
 		return collect(d, splatted, remainingMatches)
 	}
 
 	newAgg := list.New()
 
-	for el := aggregate.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		aggCandidate := el.Value.(*CandidateNode)
-		for splatEl := splatted.Front(); splatEl != nil; splatEl = splatEl.Next() {
+		for splatEl := splatted.MatchingNodes.Front(); splatEl != nil; splatEl = splatEl.Next() {
 			splatCandidate := splatEl.Value.(*CandidateNode)
 			newCandidate, err := aggCandidate.Copy()
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 
 			newCandidate.Path = nil
 
-			newCandidate, err = multiply(multiplyPreferences{AppendArrays: false})(d, newCandidate, splatCandidate)
+			newCandidate, err = multiply(multiplyPreferences{AppendArrays: false})(d, context, newCandidate, splatCandidate)
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 			newAgg.PushBack(newCandidate)
 		}
 	}
-	return collect(d, newAgg, remainingMatches)
+	return collect(d, context.ChildContext(newAgg), remainingMatches)
 
 }

pkg/yqlib/operator_collect_object_test.go

@@ -5,6 +5,30 @@ import (
 )
 
 var collectObjectOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:    true,
+		document:   "a: []",
+		expression: `.a += [{"key": "att2", "value": "val2"}]`,
+		expected: []string{
+			"D0, P[], (doc)::a: [{key: att2, value: val2}]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "",
+		expression: `.a += {"key": "att2", "value": "val2"}`,
+		expected: []string{
+			"D0, P[], ()::a:\n    key: att2\n    value: val2\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "",
+		expression: `.a += [0]`,
+		expected: []string{
+			"D0, P[], ()::a:\n    - 0\n",
+		},
+	},
 	{
 		description: `Collect empty object`,
 		document:    ``,

pkg/yqlib/operator_collect_test.go

@@ -5,6 +5,14 @@ import (
 )
 
 var collectOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:    true,
+		document:   ``,
+		expression: `.a += [0]`,
+		expected: []string{
+			"D0, P[], ()::a:\n    - 0\n",
+		},
+	},
 	{
 		description: "Collect empty",
 		document:    ``,

pkg/yqlib/operator_comments.go

@@ -13,41 +13,41 @@ type commentOpPreferences struct {
 	FootComment bool
 }
 
-func assignCommentsOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func assignCommentsOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
 	log.Debugf("AssignComments operator!")
 
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
 	preferences := expressionNode.Operation.Preferences.(commentOpPreferences)
 
 	comment := ""
 	if !expressionNode.Operation.UpdateAssign {
-		rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
-		if rhs.Front() != nil {
-			comment = rhs.Front().Value.(*CandidateNode).Node.Value
+		if rhs.MatchingNodes.Front() != nil {
+			comment = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 		}
 	}
 
-	for el := lhs.Front(); el != nil; el = el.Next() {
+	for el := lhs.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 
 		if expressionNode.Operation.UpdateAssign {
-			rhs, err := d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+			rhs, err := d.GetMatchingNodes(context.SingleReadonlyChildContext(candidate), expressionNode.Rhs)
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 
-			if rhs.Front() != nil {
-				comment = rhs.Front().Value.(*CandidateNode).Node.Value
+			if rhs.MatchingNodes.Front() != nil {
+				comment = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 			}
 		}
 
@@ -63,15 +63,15 @@ func assignCommentsOperator(d *dataTreeNavigator, matchingNodes *list.List, expr
 		}
 
 	}
-	return matchingNodes, nil
+	return context, nil
 }
 
-func getCommentsOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getCommentsOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	preferences := expressionNode.Operation.Preferences.(commentOpPreferences)
 	log.Debugf("GetComments operator!")
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		comment := ""
 		if preferences.LineComment {
@@ -87,5 +87,5 @@ func getCommentsOperator(d *dataTreeNavigator, matchingNodes *list.List, express
 		result := candidate.CreateChild(nil, node)
 		results.PushBack(result)
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_comments_test.go

@@ -62,6 +62,22 @@ var commentOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::a: cat\n\n# cat\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `a: cat`,
+		expression: `. footComment=.b.d`,
+		expected: []string{
+			"D0, P[], (doc)::a: cat\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: cat`,
+		expression: `. footComment|=.b.d`,
+		expected: []string{
+			"D0, P[], (doc)::a: cat\n",
+		},
+	},
 	{
 		description: "Remove comment",
 		document:    "a: cat # comment\nb: dog # leave this",
@@ -71,7 +87,7 @@ var commentOperatorScenarios = []expressionScenario{
 		},
 	},
 	{
-		description:    "Remove all comments",
+		description:    "Remove (strip) all comments",
 		subdescription: "Note the use of `...` to ensure key nodes are included.",
 		document:       "# hi\n\na: cat # comment\n\n# great\n\nb: # key comment",
 		expression:     `... comments=""`,

pkg/yqlib/operator_create_map.go

@@ -6,7 +6,7 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
-func createMapOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func createMapOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- createMapOperation")
 
 	//each matchingNodes entry should turn into a sequence of keys to create.
@@ -18,29 +18,29 @@ func createMapOperator(d *dataTreeNavigator, matchingNodes *list.List, expressio
 
 	sequences := list.New()
 
-	if matchingNodes.Len() > 0 {
+	if context.MatchingNodes.Len() > 0 {
 
-		for matchingNodeEl := matchingNodes.Front(); matchingNodeEl != nil; matchingNodeEl = matchingNodeEl.Next() {
+		for matchingNodeEl := context.MatchingNodes.Front(); matchingNodeEl != nil; matchingNodeEl = matchingNodeEl.Next() {
 			matchingNode := matchingNodeEl.Value.(*CandidateNode)
-			sequenceNode, err := sequenceFor(d, matchingNode, expressionNode)
+			sequenceNode, err := sequenceFor(d, context, matchingNode, expressionNode)
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 			sequences.PushBack(sequenceNode)
 		}
 	} else {
-		sequenceNode, err := sequenceFor(d, nil, expressionNode)
+		sequenceNode, err := sequenceFor(d, context, nil, expressionNode)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 		sequences.PushBack(sequenceNode)
 	}
 
-	return nodeToMap(&CandidateNode{Node: listToNodeSeq(sequences), Document: document, Path: path}), nil
+	return context.SingleChildContext(&CandidateNode{Node: listToNodeSeq(sequences), Document: document, Path: path}), nil
 
 }
 
-func sequenceFor(d *dataTreeNavigator, matchingNode *CandidateNode, expressionNode *ExpressionNode) (*CandidateNode, error) {
+func sequenceFor(d *dataTreeNavigator, context Context, matchingNode *CandidateNode, expressionNode *ExpressionNode) (*CandidateNode, error) {
 	var path []interface{}
 	var document uint = 0
 	var matches = list.New()
@@ -48,11 +48,11 @@ func sequenceFor(d *dataTreeNavigator, matchingNode *CandidateNode, expressionNo
 	if matchingNode != nil {
 		path = matchingNode.Path
 		document = matchingNode.Document
-		matches = nodeToMap(matchingNode)
+		matches.PushBack(matchingNode)
 	}
 
-	mapPairs, err := crossFunction(d, matches, expressionNode,
-		func(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+	mapPairs, err := crossFunction(d, context.ChildContext(matches), expressionNode,
+		func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
 			node := yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
 			log.Debugf("LHS:", NodeToString(lhs))
 			log.Debugf("RHS:", NodeToString(rhs))
@@ -62,12 +62,12 @@ func sequenceFor(d *dataTreeNavigator, matchingNode *CandidateNode, expressionNo
 			}
 
 			return &CandidateNode{Node: &node, Document: document, Path: path}, nil
-		})
+		}, false)
 
 	if err != nil {
 		return nil, err
 	}
-	innerList := listToNodeSeq(mapPairs)
+	innerList := listToNodeSeq(mapPairs.MatchingNodes)
 	innerList.Style = yaml.FlowStyle
 	return &CandidateNode{Node: innerList, Document: document, Path: path}, nil
 }

pkg/yqlib/operator_delete.go

@@ -1,65 +1,40 @@
 package yqlib
 
 import (
-	"container/list"
 	"fmt"
 
 	yaml "gopkg.in/yaml.v3"
 )
 
-func deleteChildOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-
-	nodesToDelete, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+func deleteChildOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	nodesToDelete, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-
-	for el := nodesToDelete.Front(); el != nil; el = el.Next() {
+	//need to iterate backwards to ensure correct indices when deleting multiple
+	for el := nodesToDelete.MatchingNodes.Back(); el != nil; el = el.Prev() {
 		candidate := el.Value.(*CandidateNode)
 
-		deleteImmediateChildOp := &Operation{
-			OperationType: deleteImmediateChildOpType,
-			Value:         candidate.Path[len(candidate.Path)-1],
+		//problem: context may already be '.a' and then I pass in '.a.a2'.
+		// should pass in .a2.
+		if candidate.Parent == nil {
+			log.Info("Could not find parent of %v", candidate.GetKey())
+			return context, nil
 		}
 
-		deleteImmediateChildOpNode := &ExpressionNode{
-			Operation: deleteImmediateChildOp,
-			Rhs:       createTraversalTree(candidate.Path[0:len(candidate.Path)-1], traversePreferences{}),
-		}
+		parentNode := candidate.Parent.Node
+		childPath := candidate.Path[len(candidate.Path)-1]
 
-		_, err := d.GetMatchingNodes(matchingNodes, deleteImmediateChildOpNode)
-		if err != nil {
-			return nil, err
-		}
-	}
-	return matchingNodes, nil
-}
-
-func deleteImmediateChildOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	parents, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
-
-	if err != nil {
-		return nil, err
-	}
-
-	childPath := expressionNode.Operation.Value
-
-	log.Debug("childPath to remove %v", childPath)
-
-	for el := parents.Front(); el != nil; el = el.Next() {
-		parent := el.Value.(*CandidateNode)
-		parentNode := unwrapDoc(parent.Node)
 		if parentNode.Kind == yaml.MappingNode {
-			deleteFromMap(parent, childPath)
+			deleteFromMap(candidate.Parent, childPath)
 		} else if parentNode.Kind == yaml.SequenceNode {
-			deleteFromArray(parent, childPath)
+			deleteFromArray(candidate.Parent, childPath)
 		} else {
-			return nil, fmt.Errorf("Cannot delete nodes from parent of tag %v", parentNode.Tag)
+			return Context{}, fmt.Errorf("Cannot delete nodes from parent of tag %v", parentNode.Tag)
 		}
-
 	}
-	return matchingNodes, nil
+	return context, nil
 }
 
 func deleteFromMap(candidate *CandidateNode, childPath interface{}) {

pkg/yqlib/operator_delete_test.go

@@ -21,6 +21,70 @@ var deleteOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::{a: {a2: frood}}\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `{a: {a1: fred, a2: frood}}`,
+		expression: `.a | del(.a1)`,
+		expected: []string{
+			"D0, P[a], (!!map)::{a2: frood}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: [1,2,3]`,
+		expression: `.a | del(.[1])`,
+		expected: []string{
+			"D0, P[a], (!!seq)::[1, 3]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[0, {a: cat, b: dog}]`,
+		expression: `.[1] | del(.a)`,
+		expected: []string{
+			"D0, P[1], (!!map)::{b: dog}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[{a: cat, b: dog}]`,
+		expression: `.[0] | del(.a)`,
+		expected: []string{
+			"D0, P[0], (!!map)::{b: dog}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[{a: {b: thing, c: frog}}]`,
+		expression: `.[0].a | del(.b)`,
+		expected: []string{
+			"D0, P[0 a], (!!map)::{c: frog}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[{a: {b: thing, c: frog}}]`,
+		expression: `.[0] | del(.a.b)`,
+		expected: []string{
+			"D0, P[0], (!!map)::{a: {c: frog}}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{a: [0, {b: thing, c: frog}]}`,
+		expression: `.a[1] | del(.b)`,
+		expected: []string{
+			"D0, P[a 1], (!!map)::{c: frog}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{a: [0, {b: thing, c: frog}]}`,
+		expression: `.a | del(.[1].b)`,
+		expected: []string{
+			"D0, P[a], (!!seq)::[0, {c: frog}]\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		document:   `{a: {a1: fred, a2: frood}}`,
@@ -37,6 +101,38 @@ var deleteOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::[1, 3]\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `a: [1,2,3]`,
+		expression: `del(.a[])`,
+		expected: []string{
+			"D0, P[], (doc)::a: []\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: [10,x,10, 10, x, 10]`,
+		expression: `del(.a[] | select(. == 10))`,
+		expected: []string{
+			"D0, P[], (doc)::a: [x, x]\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: {thing1: yep, thing2: cool, thing3: hi, b: {thing1: cool, great: huh}}`,
+		expression: `del(..)`,
+		expected: []string{
+			"D0, P[], (!!map)::{}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: {thing1: yep, thing2: cool, thing3: hi, b: {thing1: cool, great: huh}}`,
+		expression: `del(.. | select(tag == "!!map") | (.b.thing1,.thing2))`,
+		expected: []string{
+			"D0, P[], (!!map)::a: {thing1: yep, thing3: hi, b: {great: huh}}\n",
+		},
+	},
 	{
 		description: "Delete nested entry in array",
 		document:    `[{a: cat, b: dog}]`,

pkg/yqlib/operator_document_index.go

@@ -7,14 +7,14 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
-func getDocumentIndexOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getDocumentIndexOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := &yaml.Node{Kind: yaml.ScalarNode, Value: fmt.Sprintf("%v", candidate.Document), Tag: "!!int"}
 		scalar := candidate.CreateChild(nil, node)
 		results.PushBack(scalar)
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_entries.go

@@ -0,0 +1,162 @@
+package yqlib
+
+import (
+	"container/list"
+	"fmt"
+
+	yaml "gopkg.in/yaml.v3"
+)
+
+func entrySeqFor(key *yaml.Node, value *yaml.Node) *yaml.Node {
+	var keyKey = &yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: "key"}
+	var valueKey = &yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: "value"}
+
+	return &yaml.Node{
+		Kind:    yaml.MappingNode,
+		Tag:     "!!map",
+		Content: []*yaml.Node{keyKey, key, valueKey, value},
+	}
+}
+
+func toEntriesFromMap(candidateNode *CandidateNode) *CandidateNode {
+	var sequence = &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq"}
+	var entriesNode = candidateNode.CreateChild(nil, sequence)
+
+	var contents = unwrapDoc(candidateNode.Node).Content
+	for index := 0; index < len(contents); index = index + 2 {
+		key := contents[index]
+		value := contents[index+1]
+
+		sequence.Content = append(sequence.Content, entrySeqFor(key, value))
+	}
+	return entriesNode
+}
+
+func toEntriesfromSeq(candidateNode *CandidateNode) *CandidateNode {
+	var sequence = &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq"}
+	var entriesNode = candidateNode.CreateChild(nil, sequence)
+
+	var contents = unwrapDoc(candidateNode.Node).Content
+	for index := 0; index < len(contents); index = index + 1 {
+		key := &yaml.Node{Kind: yaml.ScalarNode, Tag: "!!int", Value: fmt.Sprintf("%v", index)}
+		value := contents[index]
+
+		sequence.Content = append(sequence.Content, entrySeqFor(key, value))
+	}
+	return entriesNode
+}
+
+func toEntriesOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	var results = list.New()
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+		candidateNode := unwrapDoc(candidate.Node)
+
+		switch candidateNode.Kind {
+		case yaml.MappingNode:
+			results.PushBack(toEntriesFromMap(candidate))
+
+		case yaml.SequenceNode:
+			results.PushBack(toEntriesfromSeq(candidate))
+		default:
+			if candidateNode.Tag != "!!null" {
+				return Context{}, fmt.Errorf("%v has no keys", candidate.Node.Tag)
+			}
+		}
+	}
+
+	return context.ChildContext(results), nil
+}
+
+func parseEntry(d *dataTreeNavigator, entry *yaml.Node, position int) (*yaml.Node, *yaml.Node, error) {
+	prefs := traversePreferences{DontAutoCreate: true}
+	candidateNode := &CandidateNode{Node: entry}
+
+	keyResults, err := traverseMap(Context{}, candidateNode, "key", prefs, false)
+
+	if err != nil {
+		return nil, nil, err
+	} else if keyResults.Len() != 1 {
+		return nil, nil, fmt.Errorf("Expected to find one 'key' entry but found %v in position %v", keyResults.Len(), position)
+	}
+
+	valueResults, err := traverseMap(Context{}, candidateNode, "value", prefs, false)
+
+	if err != nil {
+		return nil, nil, err
+	} else if valueResults.Len() != 1 {
+		return nil, nil, fmt.Errorf("Expected to find one 'value' entry but found %v in position %v", valueResults.Len(), position)
+	}
+
+	return keyResults.Front().Value.(*CandidateNode).Node, valueResults.Front().Value.(*CandidateNode).Node, nil
+
+}
+
+func fromEntries(d *dataTreeNavigator, candidateNode *CandidateNode) (*CandidateNode, error) {
+	var node = &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
+	var mapCandidateNode = candidateNode.CreateChild(nil, node)
+
+	var contents = unwrapDoc(candidateNode.Node).Content
+
+	for index := 0; index < len(contents); index = index + 1 {
+		key, value, err := parseEntry(d, contents[index], index)
+		if err != nil {
+			return nil, err
+		}
+
+		node.Content = append(node.Content, key, value)
+	}
+	return mapCandidateNode, nil
+}
+
+func fromEntriesOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	var results = list.New()
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+		candidateNode := unwrapDoc(candidate.Node)
+
+		switch candidateNode.Kind {
+		case yaml.SequenceNode:
+			mapResult, err := fromEntries(d, candidate)
+			if err != nil {
+				return Context{}, err
+			}
+			results.PushBack(mapResult)
+		default:
+			return Context{}, fmt.Errorf("from entries only runs against arrays")
+		}
+	}
+
+	return context.ChildContext(results), nil
+}
+
+func withEntriesOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+
+	//to_entries on the context
+	toEntries, err := toEntriesOperator(d, context, expressionNode)
+	if err != nil {
+		return Context{}, err
+	}
+
+	//run expression against entries
+	// splat toEntries and pipe it into Rhs
+	splatted, err := splat(d, toEntries, traversePreferences{})
+	if err != nil {
+		return Context{}, err
+	}
+
+	result, err := d.GetMatchingNodes(splatted, expressionNode.Rhs)
+	log.Debug("expressionNode.Rhs %v", expressionNode.Rhs.Operation.OperationType)
+	log.Debug("result %v", result)
+	if err != nil {
+		return Context{}, err
+	}
+
+	collected, err := collectOperator(d, result, expressionNode)
+	if err != nil {
+		return Context{}, err
+	}
+
+	//from_entries on the result
+	return fromEntriesOperator(d, collected, expressionNode)
+}

pkg/yqlib/operator_entries_test.go

@@ -0,0 +1,62 @@
+package yqlib
+
+import (
+	"testing"
+)
+
+var entriesOperatorScenarios = []expressionScenario{
+	{
+		description: "to_entries Map",
+		document:    `{a: 1, b: 2}`,
+		expression:  `to_entries`,
+		expected: []string{
+			"D0, P[], (!!seq)::- key: a\n  value: 1\n- key: b\n  value: 2\n",
+		},
+	},
+	{
+		description: "to_entries Array",
+		document:    `[a, b]`,
+		expression:  `to_entries`,
+		expected: []string{
+			"D0, P[], (!!seq)::- key: 0\n  value: a\n- key: 1\n  value: b\n",
+		},
+	},
+	{
+		description: "to_entries null",
+		document:    `null`,
+		expression:  `to_entries`,
+		expected:    []string{},
+	},
+	{
+		description: "from_entries map",
+		document:    `{a: 1, b: 2}`,
+		expression:  `to_entries | from_entries`,
+		expected: []string{
+			"D0, P[], (!!map)::a: 1\nb: 2\n",
+		},
+	},
+	{
+		description:    "from_entries with numeric key indexes",
+		subdescription: "from_entries always creates a map, even for numeric keys",
+		document:       `[a,b]`,
+		expression:     `to_entries | from_entries`,
+		expected: []string{
+			"D0, P[], (!!map)::0: a\n1: b\n",
+		},
+	},
+	{
+		description: "Use with_entries to update keys",
+		document:    `{a: 1, b: 2}`,
+		expression:  `with_entries(.key |= "KEY_" + .)`,
+		expected: []string{
+			"D0, P[], (!!map)::KEY_a: 1\nKEY_b: 2\n",
+		},
+	},
+}
+
+func TestEntriesOperatorScenarios(t *testing.T) {
+	for _, tt := range entriesOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	documentScenarios(t, "Entries", entriesOperatorScenarios)
+}

pkg/yqlib/operator_env.go

@@ -1,7 +1,6 @@
 package yqlib
 
 import (
-	"container/list"
 	"fmt"
 	"os"
 	"strings"
@@ -13,7 +12,7 @@ type envOpPreferences struct {
 	StringValue bool
 }
 
-func envOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func envOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	envName := expressionNode.Operation.CandidateNode.Node.Value
 	log.Debug("EnvOperator, env name:", envName)
 
@@ -29,13 +28,13 @@ func envOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *Expr
 			Value: rawValue,
 		}
 	} else if rawValue == "" {
-		return nil, fmt.Errorf("Value for env variable '%v' not provided in env()", envName)
+		return Context{}, fmt.Errorf("Value for env variable '%v' not provided in env()", envName)
 	} else {
 		var dataBucket yaml.Node
 		decoder := yaml.NewDecoder(strings.NewReader(rawValue))
 		errorReading := decoder.Decode(&dataBucket)
 		if errorReading != nil {
-			return nil, errorReading
+			return Context{}, errorReading
 		}
 		//first node is a doc
 		node = unwrapDoc(&dataBucket)
@@ -46,5 +45,5 @@ func envOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *Expr
 
 	target := &CandidateNode{Node: node}
 
-	return nodeToMap(target), nil
+	return context.SingleChildContext(target), nil
 }

pkg/yqlib/operator_equals.go

@@ -1,25 +1,54 @@
 package yqlib
 
-import (
-	"container/list"
-)
+import "gopkg.in/yaml.v3"
 
-func equalsOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func equalsOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- equalsOperation")
-	return crossFunction(d, matchingNodes, expressionNode, isEquals)
+	return crossFunction(d, context.ReadOnlyClone(), expressionNode, isEquals(false), true)
 }
 
-func isEquals(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
-	value := false
+func isEquals(flip bool) func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+	return func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+		value := false
+		log.Debugf("-- isEquals cross function")
+		if lhs == nil && rhs == nil {
+			owner := &CandidateNode{}
+			return createBooleanCandidate(owner, !flip), nil
+		} else if lhs == nil {
+			log.Debugf("lhs nil, but rhs is not")
+			rhsNode := unwrapDoc(rhs.Node)
+			value := rhsNode.Tag == "!!null"
+			if flip {
+				value = !value
+			}
+			return createBooleanCandidate(rhs, value), nil
+		} else if rhs == nil {
+			log.Debugf("lhs not nil, but rhs is")
+			lhsNode := unwrapDoc(lhs.Node)
+			value := lhsNode.Tag == "!!null"
+			if flip {
+				value = !value
+			}
+			return createBooleanCandidate(lhs, value), nil
+		}
 
-	lhsNode := unwrapDoc(lhs.Node)
-	rhsNode := unwrapDoc(rhs.Node)
+		lhsNode := unwrapDoc(lhs.Node)
+		rhsNode := unwrapDoc(rhs.Node)
 
-	if lhsNode.Tag == "!!null" {
-		value = (rhsNode.Tag == "!!null")
-	} else {
-		value = matchKey(lhsNode.Value, rhsNode.Value)
+		if lhsNode.Tag == "!!null" {
+			value = (rhsNode.Tag == "!!null")
+		} else if lhsNode.Kind == yaml.ScalarNode && rhsNode.Kind == yaml.ScalarNode {
+			value = matchKey(lhsNode.Value, rhsNode.Value)
+		}
+		log.Debugf("%v == %v ? %v", NodeToString(lhs), NodeToString(rhs), value)
+		if flip {
+			value = !value
+		}
+		return createBooleanCandidate(lhs, value), nil
 	}
-	log.Debugf("%v == %v ? %v", NodeToString(lhs), NodeToString(rhs), value)
-	return createBooleanCandidate(lhs, value), nil
+}
+
+func notEqualsOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	log.Debugf("-- notEqualsOperator")
+	return crossFunction(d, context.ReadOnlyClone(), expressionNode, isEquals(true), true)
 }

pkg/yqlib/operator_equals_test.go

@@ -5,6 +5,13 @@ import (
 )
 
 var equalsOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:    true,
+		expression: ".a == .b",
+		expected: []string{
+			"D0, P[], (!!bool)::true\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		document:   "cat",
@@ -14,6 +21,68 @@ var equalsOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!bool)::false\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   "{}",
+		expression: "(.a == .b) as $x",
+		expected: []string{
+			"D0, P[], (doc)::{}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "{}",
+		expression: ".a == .b",
+		expected: []string{
+			"D0, P[], (!!bool)::true\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "{}",
+		expression: "(.a != .b) as $x",
+		expected: []string{
+			"D0, P[], (doc)::{}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "{}",
+		expression: ".a != .b",
+		expected: []string{
+			"D0, P[], (!!bool)::false\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "{a: {b: 10}}",
+		expression: "select(.c != null)",
+		expected:   []string{},
+	},
+	{
+		skipDoc:    true,
+		document:   "{a: {b: 10}}",
+		expression: "select(.d == .c)",
+		expected: []string{
+			"D0, P[], (doc)::{a: {b: 10}}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "{a: {b: 10}}",
+		expression: "select(null == .c)",
+		expected: []string{
+			"D0, P[], (doc)::{a: {b: 10}}\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "{a: { b: {things: \"\"}, f: [1], g: [] }}",
+		expression: ".. | select(. == \"\")",
+		expected: []string{
+			"D0, P[a b things], (!!str)::\"\"\n",
+		},
+	},
 	{
 		description: "Match string",
 		document:    `[cat,goat,dog]`,
@@ -23,7 +92,18 @@ var equalsOperatorScenarios = []expressionScenario{
 			"D0, P[1], (!!bool)::true\n",
 			"D0, P[2], (!!bool)::false\n",
 		},
-	}, {
+	},
+	{
+		description: "Don't match string",
+		document:    `[cat,goat,dog]`,
+		expression:  `.[] | (. != "*at")`,
+		expected: []string{
+			"D0, P[0], (!!bool)::false\n",
+			"D0, P[1], (!!bool)::false\n",
+			"D0, P[2], (!!bool)::true\n",
+		},
+	},
+	{
 		description: "Match number",
 		document:    `[3, 4, 5]`,
 		expression:  `.[] | (. == 4)`,
@@ -32,7 +112,18 @@ var equalsOperatorScenarios = []expressionScenario{
 			"D0, P[1], (!!bool)::true\n",
 			"D0, P[2], (!!bool)::false\n",
 		},
-	}, {
+	},
+	{
+		description: "Dont match number",
+		document:    `[3, 4, 5]`,
+		expression:  `.[] | (. != 4)`,
+		expected: []string{
+			"D0, P[0], (!!bool)::true\n",
+			"D0, P[1], (!!bool)::false\n",
+			"D0, P[2], (!!bool)::true\n",
+		},
+	},
+	{
 		skipDoc:    true,
 		document:   `a: { cat: {b: apple, c: whatever}, pat: {b: banana} }`,
 		expression: `.a | (.[].b == "apple")`,
@@ -57,6 +148,22 @@ var equalsOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!bool)::true\n",
 		},
 	},
+	{
+		description: "Non exisitant key doesn't equal a value",
+		document:    "a: frog",
+		expression:  `select(.b != "thing")`,
+		expected: []string{
+			"D0, P[], (doc)::a: frog\n",
+		},
+	},
+	{
+		description: "Two non existant keys are equal",
+		document:    "a: frog",
+		expression:  `select(.b == .c)`,
+		expected: []string{
+			"D0, P[], (doc)::a: frog\n",
+		},
+	},
 }
 
 func TestEqualOperatorScenarios(t *testing.T) {

pkg/yqlib/operator_file.go

@@ -7,32 +7,32 @@ import (
 	yaml "gopkg.in/yaml.v3"
 )
 
-func getFilenameOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getFilenameOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("GetFilename")
 
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := &yaml.Node{Kind: yaml.ScalarNode, Value: candidate.Filename, Tag: "!!str"}
 		result := candidate.CreateChild(nil, node)
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }
 
-func getFileIndexOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getFileIndexOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("GetFileIndex")
 
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := &yaml.Node{Kind: yaml.ScalarNode, Value: fmt.Sprintf("%v", candidate.FileIndex), Tag: "!!int"}
 		result := candidate.CreateChild(nil, node)
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_file_test.go

@@ -21,6 +21,16 @@ var fileOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!int)::0\n",
 		},
 	},
+	{
+		description: "Get file indices of multiple documents",
+		document:    `{a: cat}`,
+		document2:   `{a: cat}`,
+		expression:  `fileIndex`,
+		expected: []string{
+			"D0, P[], (!!int)::0\n",
+			"D0, P[], (!!int)::1\n",
+		},
+	},
 	{
 		description: "Get file index alias",
 		document:    `{a: cat}`,

pkg/yqlib/operator_has.go

@@ -7,20 +7,25 @@ import (
 	yaml "gopkg.in/yaml.v3"
 )
 
-func hasOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func hasOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
 	log.Debugf("-- hasOperation")
 	var results = list.New()
 
-	rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
-	wanted := rhs.Front().Value.(*CandidateNode).Node
-	wantedKey := wanted.Value
+	rhs, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	wantedKey := "null"
+	wanted := &yaml.Node{Tag: "!!null"}
+	if rhs.MatchingNodes.Len() != 0 {
+		wanted = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node
+		wantedKey = wanted.Value
+	}
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 
 		// grab the first value
@@ -41,7 +46,7 @@ func hasOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode
 			if wanted.Tag == "!!int" {
 				var number, errParsingInt = strconv.ParseInt(wantedKey, 10, 64) // nolint
 				if errParsingInt != nil {
-					return nil, errParsingInt
+					return Context{}, errParsingInt
 				}
 				candidateHasKey = int64(len(contents)) > number
 			}
@@ -50,5 +55,5 @@ func hasOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode
 			results.PushBack(createBooleanCandidate(candidate, false))
 		}
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_has_test.go

@@ -13,6 +13,22 @@ var hasOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!bool)::true\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `a: hello`,
+		expression: `has(.b) as $c`,
+		expected: []string{
+			"D0, P[], (doc)::a: hello\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `a: hello`,
+		expression: `has(.b)`,
+		expected: []string{
+			"D0, P[], (!!bool)::false\n",
+		},
+	},
 	{
 		description: "Has map key",
 		document: `- a: "yes"
@@ -28,6 +44,15 @@ var hasOperatorScenarios = []expressionScenario{
 			"D0, P[3], (!!bool)::false\n",
 		},
 	},
+	{
+		description:    "Select, checking for existence of deep paths",
+		subdescription: "Simply pipe in parent expressions into `has`",
+		document:       "- {a: {b: {c: cat}}}\n- {a: {b: {d: dog}}}",
+		expression:     `.[] | select(.a.b | has("c"))`,
+		expected: []string{
+			"D0, P[0], (!!map)::{a: {b: {c: cat}}}\n",
+		},
+	},
 	{
 		dontFormatInputForDoc: true,
 		description:           "Has array index",

pkg/yqlib/operator_keys.go

@@ -7,12 +7,12 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
-func keysOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func keysOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- keysOperator")
 
 	var results = list.New()
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := unwrapDoc(candidate.Node)
 		var targetNode *yaml.Node
@@ -21,14 +21,14 @@ func keysOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *Exp
 		} else if node.Kind == yaml.SequenceNode {
 			targetNode = getIndicies(node)
 		} else {
-			return nil, fmt.Errorf("Cannot get keys of %v, keys only works for maps and arrays", node.Tag)
+			return Context{}, fmt.Errorf("Cannot get keys of %v, keys only works for maps and arrays", node.Tag)
 		}
 
 		result := candidate.CreateChild(nil, targetNode)
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }
 
 func getMapKeys(node *yaml.Node) *yaml.Node {

pkg/yqlib/operator_length.go

@@ -7,17 +7,21 @@ import (
 	yaml "gopkg.in/yaml.v3"
 )
 
-func lengthOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func lengthOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- lengthOperation")
 	var results = list.New()
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		targetNode := unwrapDoc(candidate.Node)
 		var length int
 		switch targetNode.Kind {
 		case yaml.ScalarNode:
-			length = len(targetNode.Value)
+			if targetNode.Tag == "!!null" {
+				length = 0
+			} else {
+				length = len(targetNode.Value)
+			}
 		case yaml.MappingNode:
 			length = len(targetNode.Content) / 2
 		case yaml.SequenceNode:
@@ -31,5 +35,5 @@ func lengthOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *E
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_length_test.go

@@ -14,6 +14,30 @@ var lengthOperatorScenarios = []expressionScenario{
 			"D0, P[a], (!!int)::3\n",
 		},
 	},
+	{
+		description: "null length",
+		document:    `{a: null}`,
+		expression:  `.a | length`,
+		expected: []string{
+			"D0, P[a], (!!int)::0\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{a: ~}`,
+		expression: `.a | length`,
+		expected: []string{
+			"D0, P[a], (!!int)::0\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{a: key no exist}`,
+		expression: `.b | length`,
+		expected: []string{
+			"D0, P[b], (!!int)::0\n",
+		},
+	},
 	{
 		description:    "Map length",
 		subdescription: "returns number of entries",

pkg/yqlib/operator_multiply.go

@@ -9,77 +9,19 @@ import (
 	yaml "gopkg.in/yaml.v3"
 )
 
-type crossFunctionCalculation func(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error)
-
-func doCrossFunc(d *dataTreeNavigator, contextList *list.List, expressionNode *ExpressionNode, calculation crossFunctionCalculation) (*list.List, error) {
-	var results = list.New()
-	lhs, err := d.GetMatchingNodes(contextList, expressionNode.Lhs)
-	if err != nil {
-		return nil, err
-	}
-	log.Debugf("crossFunction LHS len: %v", lhs.Len())
-
-	rhs, err := d.GetMatchingNodes(contextList, expressionNode.Rhs)
-
-	if err != nil {
-		return nil, err
-	}
-
-	for el := lhs.Front(); el != nil; el = el.Next() {
-		lhsCandidate := el.Value.(*CandidateNode)
-
-		for rightEl := rhs.Front(); rightEl != nil; rightEl = rightEl.Next() {
-			log.Debugf("Applying calc")
-			rhsCandidate := rightEl.Value.(*CandidateNode)
-			resultCandidate, err := calculation(d, lhsCandidate, rhsCandidate)
-			if err != nil {
-				return nil, err
-			}
-			results.PushBack(resultCandidate)
-		}
-
-	}
-	return results, nil
-}
-
-func crossFunction(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode, calculation crossFunctionCalculation) (*list.List, error) {
-	var results = list.New()
-
-	var evaluateAllTogether = true
-	for matchEl := matchingNodes.Front(); matchEl != nil; matchEl = matchEl.Next() {
-		evaluateAllTogether = evaluateAllTogether && matchEl.Value.(*CandidateNode).EvaluateTogether
-		if !evaluateAllTogether {
-			break
-		}
-	}
-	if evaluateAllTogether {
-		return doCrossFunc(d, matchingNodes, expressionNode, calculation)
-	}
-
-	for matchEl := matchingNodes.Front(); matchEl != nil; matchEl = matchEl.Next() {
-		contextList := nodeToMap(matchEl.Value.(*CandidateNode))
-		innerResults, err := doCrossFunc(d, contextList, expressionNode, calculation)
-		if err != nil {
-			return nil, err
-		}
-		results.PushBackList(innerResults)
-	}
-
-	return results, nil
-}
-
 type multiplyPreferences struct {
-	AppendArrays  bool
-	TraversePrefs traversePreferences
+	AppendArrays    bool
+	DeepMergeArrays bool
+	TraversePrefs   traversePreferences
 }
 
-func multiplyOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func multiplyOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- MultiplyOperator")
-	return crossFunction(d, matchingNodes, expressionNode, multiply(expressionNode.Operation.Preferences.(multiplyPreferences)))
+	return crossFunction(d, context, expressionNode, multiply(expressionNode.Operation.Preferences.(multiplyPreferences)), false)
 }
 
-func multiply(preferences multiplyPreferences) func(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
-	return func(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+func multiply(preferences multiplyPreferences) func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+	return func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
 		lhs.Node = unwrapDoc(lhs.Node)
 		rhs.Node = unwrapDoc(rhs.Node)
 		log.Debugf("Multipling LHS: %v", lhs.Node.Tag)
@@ -89,18 +31,39 @@ func multiply(preferences multiplyPreferences) func(d *dataTreeNavigator, lhs *C
 			(lhs.Node.Kind == yaml.SequenceNode && rhs.Node.Kind == yaml.SequenceNode) {
 
 			var newBlank = lhs.CreateChild(nil, &yaml.Node{})
-			var newThing, err = mergeObjects(d, newBlank, lhs, multiplyPreferences{})
+
+			var newThing, err = mergeObjects(d, context, newBlank, lhs, multiplyPreferences{})
 			if err != nil {
 				return nil, err
 			}
-			return mergeObjects(d, newThing, rhs, preferences)
+			return mergeObjects(d, context, newThing, rhs, preferences)
 		} else if lhs.Node.Tag == "!!int" && rhs.Node.Tag == "!!int" {
 			return multiplyIntegers(lhs, rhs)
+		} else if (lhs.Node.Tag == "!!int" || lhs.Node.Tag == "!!float") && (rhs.Node.Tag == "!!int" || rhs.Node.Tag == "!!float") {
+			return multiplyFloats(lhs, rhs)
 		}
 		return nil, fmt.Errorf("Cannot multiply %v with %v", lhs.Node.Tag, rhs.Node.Tag)
 	}
 }
 
+func multiplyFloats(lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+	target := lhs.CreateChild(nil, &yaml.Node{})
+	target.Node.Kind = yaml.ScalarNode
+	target.Node.Style = lhs.Node.Style
+	target.Node.Tag = "!!float"
+
+	lhsNum, err := strconv.ParseFloat(lhs.Node.Value, 64)
+	if err != nil {
+		return nil, err
+	}
+	rhsNum, err := strconv.ParseFloat(rhs.Node.Value, 64)
+	if err != nil {
+		return nil, err
+	}
+	target.Node.Value = fmt.Sprintf("%v", lhsNum*rhsNum)
+	return target, nil
+}
+
 func multiplyIntegers(lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
 	target := lhs.CreateChild(nil, &yaml.Node{})
 	target.Node.Kind = yaml.ScalarNode
@@ -119,14 +82,14 @@ func multiplyIntegers(lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, e
 	return target, nil
 }
 
-func mergeObjects(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode, preferences multiplyPreferences) (*CandidateNode, error) {
+func mergeObjects(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode, preferences multiplyPreferences) (*CandidateNode, error) {
 	shouldAppendArrays := preferences.AppendArrays
 	var results = list.New()
 
 	// shouldn't recurse arrays if appending
 	prefs := recursiveDescentPreferences{RecurseArray: !shouldAppendArrays,
-		TraversePreferences: traversePreferences{DontFollowAlias: true}}
-	err := recursiveDecent(d, results, nodeToMap(rhs), prefs)
+		TraversePreferences: traversePreferences{DontFollowAlias: true, IncludeMapKeys: true}}
+	err := recursiveDecent(d, results, context.SingleChildContext(rhs), prefs)
 	if err != nil {
 		return nil, err
 	}
@@ -137,7 +100,12 @@ func mergeObjects(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode,
 	}
 
 	for el := results.Front(); el != nil; el = el.Next() {
-		err := applyAssignment(d, pathIndexToStartFrom, lhs, el.Value.(*CandidateNode), preferences)
+		candidate := el.Value.(*CandidateNode)
+		if candidate.Node.Tag == "!!merge" {
+			continue
+		}
+
+		err := applyAssignment(d, context, pathIndexToStartFrom, lhs, candidate, preferences)
 		if err != nil {
 			return nil, err
 		}
@@ -145,24 +113,25 @@ func mergeObjects(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode,
 	return lhs, nil
 }
 
-func applyAssignment(d *dataTreeNavigator, pathIndexToStartFrom int, lhs *CandidateNode, rhs *CandidateNode, preferences multiplyPreferences) error {
+func applyAssignment(d *dataTreeNavigator, context Context, pathIndexToStartFrom int, lhs *CandidateNode, rhs *CandidateNode, preferences multiplyPreferences) error {
 	shouldAppendArrays := preferences.AppendArrays
-	log.Debugf("merge - applyAssignment lhs %v, rhs: %v", NodeToString(lhs), NodeToString(rhs))
+	log.Debugf("merge - applyAssignment lhs %v, rhs: %v", lhs.GetKey(), rhs.GetKey())
 
 	lhsPath := rhs.Path[pathIndexToStartFrom:]
 
 	assignmentOp := &Operation{OperationType: assignAttributesOpType}
-	if rhs.Node.Kind == yaml.ScalarNode || rhs.Node.Kind == yaml.AliasNode {
+	if shouldAppendArrays && rhs.Node.Kind == yaml.SequenceNode {
+		assignmentOp.OperationType = addAssignOpType
+	} else if !preferences.DeepMergeArrays && rhs.Node.Kind == yaml.SequenceNode ||
+		(rhs.Node.Kind == yaml.ScalarNode || rhs.Node.Kind == yaml.AliasNode) {
 		assignmentOp.OperationType = assignOpType
 		assignmentOp.UpdateAssign = false
-	} else if shouldAppendArrays && rhs.Node.Kind == yaml.SequenceNode {
-		assignmentOp.OperationType = addAssignOpType
 	}
 	rhsOp := &Operation{OperationType: valueOpType, CandidateNode: rhs}
 
-	assignmentOpNode := &ExpressionNode{Operation: assignmentOp, Lhs: createTraversalTree(lhsPath, preferences.TraversePrefs), Rhs: &ExpressionNode{Operation: rhsOp}}
+	assignmentOpNode := &ExpressionNode{Operation: assignmentOp, Lhs: createTraversalTree(lhsPath, preferences.TraversePrefs, rhs.IsMapKey), Rhs: &ExpressionNode{Operation: rhsOp}}
 
-	_, err := d.GetMatchingNodes(nodeToMap(lhs), assignmentOpNode)
+	_, err := d.GetMatchingNodes(context.SingleChildContext(lhs), assignmentOpNode)
 
 	return err
 }

pkg/yqlib/operator_multiply_test.go

@@ -4,7 +4,63 @@ import (
 	"testing"
 )
 
+var doc1 = `list:
+# Hi this is a comment.
+# Hello this is another comment.
+- "abc"`
+
+var doc2 = `list2:
+# This is yet another comment.
+# Indeed this is yet another comment.
+- "123"`
+
+var docExpected = `D0, P[], (!!map)::list:
+    # Hi this is a comment.
+    # Hello this is another comment.
+    - "abc"
+list2:
+    # This is yet another comment.
+    # Indeed this is yet another comment.
+    - "123"
+`
+
+var mergeArraysObjectKeysText = `It's a complex command, the trickyness comes from needing to have the right context in the expressions.
+First we save the second array into a variable '$two' which lets us reference it later.
+We then need to update the first array. We will use the relative update (|=) because we need to update relative to the current element of the array in the LHS in the RHS expression. 
+We set the current element of the first array as $cur. Now we multiply (merge) $cur with the matching entry in $two, by passing $two through a select filter.
+`
+
 var multiplyOperatorScenarios = []expressionScenario{
+	{
+		description: "Multiply integers",
+		expression:  `3 * 4`,
+		expected: []string{
+			"D0, P[], (!!int)::12\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   doc1,
+		document2:  doc2,
+		expression: `select(fi == 0) * select(fi == 1)`,
+		expected: []string{
+			docExpected,
+		},
+	},
+	{
+		skipDoc:    true,
+		expression: `3 * 4.5`,
+		expected: []string{
+			"D0, P[], (!!float)::13.5\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		expression: `4.5 * 3`,
+		expected: []string{
+			"D0, P[], (!!float)::13.5\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		document:   `{a: {also: [1]}, b: {also: me}}`,
@@ -13,6 +69,30 @@ var multiplyOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!map)::{a: {also: me}, b: {also: me}}\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   "# b\nb:\n  # a\n  a: cat",
+		expression: "{} * .",
+		expected: []string{
+			"D0, P[], (!!map)::# b\nb:\n    # a\n    a: cat\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   "# b\nb:\n  # a\n  a: cat",
+		expression: ". * {}",
+		expected: []string{
+			"D0, P[], (!!map)::# b\nb:\n    # a\n    a: cat\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{a: &a { b: &b { c: &c cat } } }`,
+		expression: `{} * .`,
+		expected: []string{
+			"D0, P[], (!!map)::{a: &a {b: &b {c: &c cat}}}\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		document:   `{a: 2, b: 5}`,
@@ -92,7 +172,7 @@ var multiplyOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   `{a: {things: great}, b: {also: me}}`,
-		expression: `. * {"a":.b}`,
+		expression: `. * {"a": .b}`,
 		expected: []string{
 			"D0, P[], (!!map)::{a: {things: great, also: me}, b: {also: me}}\n",
 		},
@@ -100,16 +180,10 @@ var multiplyOperatorScenarios = []expressionScenario{
 	{
 		description:           "Merge keeps style of LHS",
 		dontFormatInputForDoc: true,
-		document: `a: {things: great}
-b:
-  also: "me"
-`,
-		expression: `. * {"a":.b}`,
+		document:              "a: {things: great}\nb:\n  also: \"me\"",
+		expression:            `. * {"a":.b}`,
 		expected: []string{
-			`D0, P[], (!!map)::a: {things: great, also: "me"}
-b:
-    also: "me"
-`,
+			"D0, P[], (!!map)::a: {things: great, also: \"me\"}\nb:\n    also: \"me\"\n",
 		},
 	},
 	{
@@ -139,7 +213,7 @@ b:
 	{
 		skipDoc:    true,
 		document:   `{a: [{thing: one}], b: [{missing: two, thing: two}]}`,
-		expression: `.a *? .b`,
+		expression: `.a *?d .b`,
 		expected: []string{
 			"D0, P[a], (!!seq)::[{thing: two}]\n",
 		},
@@ -168,6 +242,25 @@ b:
 			"D0, P[a], (!!map)::{thing: [1, 2, 3, 4]}\n",
 		},
 	},
+	{
+		description:    "Merge, deeply merging arrays",
+		subdescription: "Merging arrays deeply means arrays are merge like objects, with indexes as their key. In this case, we merge the first item in the array, and do nothing with the second.",
+		document:       `{a: [{name: fred, age: 12}, {name: bob, age: 32}], b: [{name: fred, age: 34}]}`,
+		expression:     `.a *d .b`,
+		expected: []string{
+			"D0, P[a], (!!seq)::[{name: fred, age: 34}, {name: bob, age: 32}]\n",
+		},
+	},
+	{
+		description:    "Merge arrays of objects together, matching on a key",
+		subdescription: mergeArraysObjectKeysText,
+		document:       `[{a: apple, b: appleB}, {a: kiwi, b: kiwiB}, {a: banana, b: bananaB}]`,
+		document2:      `[{a: banana, c: bananaC}, {a: apple, b: appleB2}, {a: dingo, c: dingoC}]`,
+		expression:     `(select(fi==1) | .[]) as $two | select(fi==0) | .[] |= (. as $cur |  $cur * ($two | select(.a == $cur.a)))`,
+		expected: []string{
+			"D0, P[], (doc)::[{a: apple, b: appleB2}, {a: kiwi, b: kiwiB}, {a: banana, b: bananaB, c: bananaC}]\n",
+		},
+	},
 	{
 		description: "Merge to prefix an element",
 		document:    `{a: cat, b: dog}`,
@@ -206,5 +299,5 @@ func TestMultiplyOperatorScenarios(t *testing.T) {
 	for _, tt := range multiplyOperatorScenarios {
 		testScenario(t, &tt)
 	}
-	documentScenarios(t, "Multiply", multiplyOperatorScenarios)
+	documentScenarios(t, "Multiply (Merge)", multiplyOperatorScenarios)
 }

pkg/yqlib/operator_path.go

@@ -16,12 +16,12 @@ func createPathNodeFor(pathElement interface{}) *yaml.Node {
 	}
 }
 
-func getPathOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getPathOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("GetPath")
 
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := &yaml.Node{Kind: yaml.SequenceNode, Tag: "!!seq"}
 
@@ -35,5 +35,5 @@ func getPathOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionN
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_pipe.go

@@ -1,11 +1,18 @@
 package yqlib
 
-import "container/list"
+func pipeOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
-func pipeOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
+	//lhs may update the variable context, we should pass that into the RHS
+	// BUT we still return the original context back (see jq)
+	// https://stedolan.github.io/jq/manual/#Variable/SymbolicBindingOperator:...as$identifier|...
+
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-	return d.GetMatchingNodes(lhs, expressionNode.Rhs)
+	rhs, err := d.GetMatchingNodes(lhs, expressionNode.Rhs)
+	if err != nil {
+		return Context{}, err
+	}
+	return context.ChildContext(rhs.MatchingNodes), nil
 }

pkg/yqlib/operator_recursive_descent.go

@@ -11,20 +11,20 @@ type recursiveDescentPreferences struct {
 	RecurseArray        bool
 }
 
-func recursiveDescentOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func recursiveDescentOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	var results = list.New()
 
 	preferences := expressionNode.Operation.Preferences.(recursiveDescentPreferences)
-	err := recursiveDecent(d, results, matchMap, preferences)
+	err := recursiveDecent(d, results, context, preferences)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }
 
-func recursiveDecent(d *dataTreeNavigator, results *list.List, matchMap *list.List, preferences recursiveDescentPreferences) error {
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+func recursiveDecent(d *dataTreeNavigator, results *list.List, context Context, preferences recursiveDescentPreferences) error {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 
 		candidate.Node = unwrapDoc(candidate.Node)
@@ -35,7 +35,7 @@ func recursiveDecent(d *dataTreeNavigator, results *list.List, matchMap *list.Li
 		if candidate.Node.Kind != yaml.AliasNode && len(candidate.Node.Content) > 0 &&
 			(preferences.RecurseArray || candidate.Node.Kind != yaml.SequenceNode) {
 
-			children, err := splat(d, nodeToMap(candidate), preferences.TraversePreferences)
+			children, err := splat(d, context.SingleChildContext(candidate), preferences.TraversePreferences)
 
 			if err != nil {
 				return err

pkg/yqlib/operator_reduce.go

@@ -0,0 +1,59 @@
+package yqlib
+
+import (
+	"container/list"
+	"fmt"
+)
+
+func reduceOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	log.Debugf("-- reduceOp")
+	//.a as $var reduce (0; . + $var)
+	//lhs is the assignment operator
+	//rhs is the reduce block
+	// '.' refers to the current accumulator, initialised to 0
+	// $var references a single element from the .a
+
+	//ensure lhs is actually an assignment
+	//and rhs is a block (empty)
+	if expressionNode.Lhs.Operation.OperationType != assignVariableOpType {
+		return Context{}, fmt.Errorf("reduce must be given a variables assignment, got %v instead", expressionNode.Lhs.Operation.OperationType.Type)
+	} else if expressionNode.Rhs.Operation.OperationType != blockOpType {
+		return Context{}, fmt.Errorf("reduce must be given a block, got %v instead", expressionNode.Rhs.Operation.OperationType.Type)
+	}
+
+	arrayExpNode := expressionNode.Lhs.Lhs
+	array, err := d.GetMatchingNodes(context, arrayExpNode)
+
+	log.Debugf("array of %v things", array.MatchingNodes.Len())
+
+	if err != nil {
+		return Context{}, err
+	}
+
+	variableName := expressionNode.Lhs.Rhs.Operation.StringValue
+
+	initExp := expressionNode.Rhs.Lhs
+
+	accum, err := d.GetMatchingNodes(context, initExp)
+	if err != nil {
+		return Context{}, err
+	}
+
+	log.Debugf("with variable %v", variableName)
+
+	blockExp := expressionNode.Rhs.Rhs
+	for el := array.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+		log.Debugf("REDUCING WITH %v", NodeToString(candidate))
+		l := list.New()
+		l.PushBack(candidate)
+		accum.SetVariable(variableName, l)
+
+		accum, err = d.GetMatchingNodes(accum, blockExp)
+		if err != nil {
+			return Context{}, err
+		}
+	}
+
+	return accum, nil
+}

pkg/yqlib/operator_reduce_test.go

@@ -0,0 +1,40 @@
+package yqlib
+
+import (
+	"testing"
+)
+
+var reduceOperatorScenarios = []expressionScenario{
+	{
+		description: "Sum numbers",
+		document:    `[10,2, 5, 3]`,
+		expression:  `.[] as $item ireduce (0; . + $item)`,
+		expected: []string{
+			"D0, P[], (!!int)::20\n",
+		},
+	},
+	{
+		description: "Merge all yaml files together",
+		document:    `a: cat`,
+		document2:   `b: dog`,
+		expression:  `. as $item ireduce ({}; . * $item )`,
+		expected: []string{
+			"D0, P[], (!!map)::a: cat\nb: dog\n",
+		},
+	},
+	{
+		description: "Convert an array to an object",
+		document:    `[{name: Cathy, has: apples},{name: Bob, has: bananas}]`,
+		expression:  `.[] as $item ireduce ({}; .[$item | .name] = ($item | .has) )`,
+		expected: []string{
+			"D0, P[], (!!map)::Cathy: apples\nBob: bananas\n",
+		},
+	},
+}
+
+func TestReduceOperatorScenarios(t *testing.T) {
+	for _, tt := range reduceOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	documentScenarios(t, "Reduce", reduceOperatorScenarios)
+}

pkg/yqlib/operator_select.go

@@ -4,28 +4,29 @@ import (
 	"container/list"
 )
 
-func selectOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func selectOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
 	log.Debugf("-- selectOperation")
 	var results = list.New()
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
-
-		rhs, err := d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context.SingleReadonlyChildContext(candidate), expressionNode.Rhs)
 
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
 		// grab the first value
-		first := rhs.Front()
+		first := rhs.MatchingNodes.Front()
 
 		if first != nil {
 			result := first.Value.(*CandidateNode)
+			log.Debugf("result %v", NodeToString(result))
 			includeResult, errDecoding := isTruthy(result)
+			log.Debugf("isTruthy %v", includeResult)
 			if errDecoding != nil {
-				return nil, errDecoding
+				return Context{}, errDecoding
 			}
 
 			if includeResult {
@@ -33,5 +34,5 @@ func selectOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNo
 			}
 		}
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_select_test.go

@@ -14,6 +14,24 @@ var selectOperatorScenarios = []expressionScenario{
 			"D0, P[1], (!!str)::goat\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   "a: hello",
+		document2:  "b: world",
+		expression: `select(.a == "hello" or .b == "world")`,
+		expected: []string{
+			"D0, P[], (doc)::a: hello\n",
+			"D0, P[], (doc)::b: world\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `[{animal: cat, legs: {cool: true}}, {animal: fish}]`,
+		expression: `(.[] | select(.legs.cool == true).canWalk) = true | (.[] | .alive.things) = "yes"`,
+		expected: []string{
+			"D0, P[], (doc)::[{animal: cat, legs: {cool: true}, canWalk: true, alive: {things: yes}}, {animal: fish, alive: {things: yes}}]\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		document:   `[hot, fot, dog]`,

pkg/yqlib/operator_self.go

@@ -1,7 +1,5 @@
 package yqlib
 
-import "container/list"
-
-func selfOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	return matchMap, nil
+func selfOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	return context, nil
 }

pkg/yqlib/operator_sort_keys.go

@@ -1,33 +1,32 @@
 package yqlib
 
 import (
-	"container/list"
 	"sort"
 
 	yaml "gopkg.in/yaml.v3"
 )
 
-func sortKeysOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func sortKeysOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
-		rhs, err := d.GetMatchingNodes(nodeToMap(candidate), expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context.SingleReadonlyChildContext(candidate), expressionNode.Rhs)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
-		for childEl := rhs.Front(); childEl != nil; childEl = childEl.Next() {
+		for childEl := rhs.MatchingNodes.Front(); childEl != nil; childEl = childEl.Next() {
 			node := unwrapDoc(childEl.Value.(*CandidateNode).Node)
 			if node.Kind == yaml.MappingNode {
 				sortKeys(node)
 			}
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 		}
 
 	}
-	return matchingNodes, nil
+	return context, nil
 }
 
 func sortKeys(node *yaml.Node) {

pkg/yqlib/operator_sort_keys_test.go

@@ -13,6 +13,14 @@ var sortKeysOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::{a: blah, b: bing, c: frog}\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `{c: frog}`,
+		expression: `sortKeys(.d)`,
+		expected: []string{
+			"D0, P[], (doc)::{c: frog}\n",
+		},
+	},
 	{
 		description:    "Sort keys recursively",
 		subdescription: "Note the array elements are left unsorted, but maps inside arrays are sorted",

pkg/yqlib/operator_split_document.go

@@ -1,18 +1,14 @@
 package yqlib
 
-import (
-	"container/list"
-)
-
-func splitDocumentOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func splitDocumentOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- splitDocumentOperator")
 
 	var index uint = 0
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		candidate.Document = index
 		index = index + 1
 	}
 
-	return matchMap, nil
+	return context, nil
 }

pkg/yqlib/operator_strings.go

@@ -3,37 +3,103 @@ package yqlib
 import (
 	"container/list"
 	"fmt"
+	"regexp"
 	"strings"
 
 	"gopkg.in/yaml.v3"
 )
 
-func joinStringOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	log.Debugf("-- joinStringOperator")
-	joinStr := ""
+func getSubstituteParameters(d *dataTreeNavigator, block *ExpressionNode, context Context) (string, string, error) {
+	regEx := ""
+	replacementText := ""
 
-	rhs, err := d.GetMatchingNodes(matchMap, expressionNode.Rhs)
+	regExNodes, err := d.GetMatchingNodes(context.ReadOnlyClone(), block.Lhs)
 	if err != nil {
-		return nil, err
+		return "", "", err
 	}
-	if rhs.Front() != nil {
-		joinStr = rhs.Front().Value.(*CandidateNode).Node.Value
+	if regExNodes.MatchingNodes.Front() != nil {
+		regEx = regExNodes.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
+	}
+
+	log.Debug("regEx %v", regEx)
+
+	replacementNodes, err := d.GetMatchingNodes(context, block.Rhs)
+	if err != nil {
+		return "", "", err
+	}
+	if replacementNodes.MatchingNodes.Front() != nil {
+		replacementText = replacementNodes.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
+	}
+
+	return regEx, replacementText, nil
+}
+
+func substitute(original string, regex *regexp.Regexp, replacement string) *yaml.Node {
+	replacedString := regex.ReplaceAllString(original, replacement)
+	return &yaml.Node{Kind: yaml.ScalarNode, Value: replacedString, Tag: "!!str"}
+}
+
+func substituteStringOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	//rhs  block operator
+	//lhs of block = regex
+	//rhs of block = replacement expression
+	block := expressionNode.Rhs
+
+	regExStr, replacementText, err := getSubstituteParameters(d, block, context)
+
+	if err != nil {
+		return Context{}, err
+	}
+
+	regEx, err := regexp.Compile(regExStr)
+	if err != nil {
+		return Context{}, err
 	}
 
 	var results = list.New()
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+		node := unwrapDoc(candidate.Node)
+		if node.Tag != "!!str" {
+			return Context{}, fmt.Errorf("cannot substitute with %v, can only substitute strings. Hint: Most often you'll want to use '|=' over '=' for this operation.", node.Tag)
+		}
+
+		targetNode := substitute(node.Value, regEx, replacementText)
+		result := candidate.CreateChild(nil, targetNode)
+		results.PushBack(result)
+	}
+
+	return context.ChildContext(results), nil
+
+}
+
+func joinStringOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	log.Debugf("-- joinStringOperator")
+	joinStr := ""
+
+	rhs, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
+	if err != nil {
+		return Context{}, err
+	}
+	if rhs.MatchingNodes.Front() != nil {
+		joinStr = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
+	}
+
+	var results = list.New()
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := unwrapDoc(candidate.Node)
 		if node.Kind != yaml.SequenceNode {
-			return nil, fmt.Errorf("Cannot join with %v, can only join arrays of scalars", node.Tag)
+			return Context{}, fmt.Errorf("cannot join with %v, can only join arrays of scalars", node.Tag)
 		}
 		targetNode := join(node.Content, joinStr)
 		result := candidate.CreateChild(nil, targetNode)
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }
 
 func join(content []*yaml.Node, joinStr string) *yaml.Node {
@@ -49,35 +115,35 @@ func join(content []*yaml.Node, joinStr string) *yaml.Node {
 	return &yaml.Node{Kind: yaml.ScalarNode, Value: strings.Join(stringsToJoin, joinStr), Tag: "!!str"}
 }
 
-func splitStringOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func splitStringOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- splitStringOperator")
 	splitStr := ""
 
-	rhs, err := d.GetMatchingNodes(matchMap, expressionNode.Rhs)
+	rhs, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Rhs)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-	if rhs.Front() != nil {
-		splitStr = rhs.Front().Value.(*CandidateNode).Node.Value
+	if rhs.MatchingNodes.Front() != nil {
+		splitStr = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 	}
 
 	var results = list.New()
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 		node := unwrapDoc(candidate.Node)
 		if node.Tag == "!!null" {
 			continue
 		}
 		if node.Tag != "!!str" {
-			return nil, fmt.Errorf("Cannot split %v, can only split strings", node.Tag)
+			return Context{}, fmt.Errorf("Cannot split %v, can only split strings", node.Tag)
 		}
 		targetNode := split(node.Value, splitStr)
 		result := candidate.CreateChild(nil, targetNode)
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }
 
 func split(value string, spltStr string) *yaml.Node {

pkg/yqlib/operator_strings_test.go

@@ -13,6 +13,24 @@ var stringsOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!str)::cat; meow; 1; ; true\n",
 		},
 	},
+	{
+		description:    "Substitute / Replace string",
+		subdescription: "This uses golang regex, described [here](https://github.com/google/re2/wiki/Syntax)\nNote the use of `|=` to run in context of the current string value.",
+		document:       `a: dogs are great`,
+		expression:     `.a |= sub("dogs", "cats")`,
+		expected: []string{
+			"D0, P[], (doc)::a: cats are great\n",
+		},
+	},
+	{
+		description:    "Substitute / Replace string with regex",
+		subdescription: "This uses golang regex, described [here](https://github.com/google/re2/wiki/Syntax)\nNote the use of `|=` to run in context of the current string value.",
+		document:       "a: cat\nb: heat",
+		expression:     `.[] |= sub("(a)", "${1}r")`,
+		expected: []string{
+			"D0, P[], (doc)::a: cart\nb: heart\n",
+		},
+	},
 	{
 		description: "Split strings",
 		document:    `"cat; meow; 1; ; true"`,