v4.5.0

Allows more sophisticated functionality

.github/FUNDING.yml

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

.github/workflows/release.yml

@@ -40,17 +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

README.md

@@ -90,7 +90,7 @@ docker run --rm -v "${PWD}":/workdir mikefarah/yq <command> [flags] [expression
 #### Run commands interactively:
 
 ```bash
-docker run --rm -it -v "${PWD}":/workdir mikefarah/yq sh
+docker run --rm -it -v "${PWD}":/workdir --entrypoint sh mikefarah/yq
 ```
 
 It can be useful to have a bash function to avoid typing the whole docker command:
@@ -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

cmd/version.go

@@ -11,7 +11,7 @@ var (
 	GitDescribe string
 
 	// Version is main version number that is being run at the moment.
-	Version = "4.4.0"
+	Version = "4.5.0"
 
 	// 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:4.4.0
+FROM mikefarah/yq:4.5.0
 
 COPY entrypoint.sh /entrypoint.sh
 

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

@@ -15,6 +15,9 @@ type CandidateNode struct {
 	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
 }
 
 func (n *CandidateNode) GetKey() string {

pkg/yqlib/context.go

@@ -0,0 +1,44 @@
+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) 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
+}

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

@@ -209,3 +209,15 @@ will output
 a: 4
 ```
 
+## Add to null
+Adding to null simply returns the rhs
+
+Running
+```bash
+yq eval --null-input 'null + "cat"'
+```
+will output
+```yaml
+cat
+```
+

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.md

@@ -1,13 +1,14 @@
-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 only objects are supported, which have the effect of merging the RHS into the LHS.
 
-Upcoming versions of `yq` will add support for other types of multiplication (numbers, strings).
-
-To concatenate when merging objects, use the `*+` form (see examples below). This will recursively merge objects, appending arrays when it encounters them.
+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.
+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/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/headers/Multiply.md

@@ -1,13 +1,14 @@
-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 only objects are supported, which have the effect of merging the RHS into the LHS.
 
-Upcoming versions of `yq` will add support for other types of multiplication (numbers, strings).
-
-To concatenate when merging objects, use the `*+` form (see examples below). This will recursively merge objects, appending arrays when it encounters them.
+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.
+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/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/encoder.go

@@ -76,6 +76,8 @@ func mapKeysToStrings(node *yaml.Node) {
 
 func NewJsonEncoder(destination io.Writer, indent int) Encoder {
 	var encoder = json.NewEncoder(destination)
+	encoder.SetEscapeHTML(false) // do not escape html chars e.g. &, <, >
+
 	var indentString = ""
 
 	for index := 0; index < indent; index++ {
@@ -153,11 +155,15 @@ func (o *orderedMap) UnmarshalJSON(data []byte) error {
 }
 
 func (o orderedMap) MarshalJSON() ([]byte, error) {
-	if o.kv == nil {
-		return json.Marshal(o.altVal)
-	}
 	buf := new(bytes.Buffer)
 	enc := json.NewEncoder(buf)
+	enc.SetEscapeHTML(false) // do not escape html chars e.g. &, <, >
+	if o.kv == nil {
+		if err := enc.Encode(o.altVal); err != nil {
+			return nil, err
+		}
+		return buf.Bytes(), nil
+	}
 	buf.WriteByte('{')
 	for idx, el := range o.kv {
 		if err := enc.Encode(el.K); err != nil {

pkg/yqlib/encoder_test.go

@@ -9,29 +9,11 @@ import (
 	"github.com/mikefarah/yq/v4/test"
 )
 
-var sampleYaml = `zabbix: winner
-apple: great
-banana:
-- {cobra: kai, angus: bob}
-`
-
-var expectedJson = `{
-  "zabbix": "winner",
-  "apple": "great",
-  "banana": [
-    {
-      "cobra": "kai",
-      "angus": "bob"
-    }
-  ]
-}
-`
-
-func TestJsonEncoderPreservesObjectOrder(t *testing.T) {
+func yamlToJson(sampleYaml string, indent int) string {
 	var output bytes.Buffer
 	writer := bufio.NewWriter(&output)
 
-	var jsonEncoder = NewJsonEncoder(writer, 2)
+	var jsonEncoder = NewJsonEncoder(writer, indent)
 	inputs, err := readDocuments(strings.NewReader(sampleYaml), "sample.yml", 0)
 	if err != nil {
 		panic(err)
@@ -42,6 +24,33 @@ func TestJsonEncoderPreservesObjectOrder(t *testing.T) {
 		panic(err)
 	}
 	writer.Flush()
-	test.AssertResult(t, expectedJson, output.String())
 
+	return strings.TrimSuffix(output.String(), "\n")
+}
+
+func TestJsonEncoderPreservesObjectOrder(t *testing.T) {
+	var sampleYaml = `zabbix: winner
+apple: great
+banana:
+- {cobra: kai, angus: bob}
+`
+	var expectedJson = `{
+  "zabbix": "winner",
+  "apple": "great",
+  "banana": [
+    {
+      "cobra": "kai",
+      "angus": "bob"
+    }
+  ]
+}`
+	var actualJson = yamlToJson(sampleYaml, 2)
+	test.AssertResult(t, expectedJson, actualJson)
+}
+
+func TestJsonEncoderDoesNotEscapeHTMLChars(t *testing.T) {
+	var sampleYaml = `build: "( ./lint && ./format && ./compile ) < src.code"`
+	var expectedJson = `{"build":"( ./lint && ./format && ./compile ) < src.code"}`
+	var actualJson = yamlToJson(sampleYaml, 0)
+	test.AssertResult(t, expectedJson, actualJson)
 }

pkg/yqlib/expression_parser.go

@@ -70,7 +70,7 @@ 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 +43,23 @@ 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]
+			log.Debugf("deleteing open bracket from opstack")
+
 			//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}})
+			result = append(result, &Operation{OperationType: collectOperator})
+			log.Debugf("put collect onto the result")
+			result = append(result, &Operation{OperationType: shortPipeOpType})
+			log.Debugf("put shortpipe onto the result")
+
 		case closeBracket:
 			for len(opStack) > 0 && opStack[len(opStack)-1].TokenType != openBracket {
 				opStack, result = popOpToResult(opStack, result)
@@ -73,11 +75,12 @@ 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 +12,65 @@ var pathTests = []struct {
 	expectedTokens  []interface{}
 	expectedPostFix []interface{}
 }{
+	{
+		`.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"),
+	},
 	{
 		`.[]`,
-		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", "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", "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 +94,18 @@ 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 +138,9 @@ 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 +192,6 @@ 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 +205,7 @@ 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

@@ -34,9 +34,11 @@ type token struct {
 
 }
 
-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 "("
@@ -180,6 +182,19 @@ func stringValue(wrapped bool) lex.Action {
 	}
 }
 
+func getVariableOpToken() lex.Action {
+	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
+		value := string(m.Bytes)
+
+		value = value[1 : len(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)
@@ -304,6 +319,8 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`\*[\+|\?]*`), multiplyWithPrefs())
 	lexer.Add([]byte(`\+`), opToken(addOpType))
 	lexer.Add([]byte(`\+=`), opToken(addAssignOpType))
+	lexer.Add([]byte(`\$[a-zA-Z_-0-9]+`), getVariableOpToken())
+	lexer.Add([]byte(`as`), opToken(assignVariableOpType))
 
 	err := lexer.Compile()
 	if err != nil {
@@ -339,7 +356,7 @@ 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 {
@@ -369,6 +386,12 @@ func (p *expressionTokeniserImpl) handleToken(tokens []*token, index int, postPr
 		//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 {
+			op := &Operation{OperationType: selfReferenceOpType, StringValue: "SELF"}
+			postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
+		}
+
 		op := &Operation{OperationType: traverseArrayOpType, StringValue: "TRAVERSE_ARRAY"}
 		postProcessedTokens = append(postProcessedTokens, &token{TokenType: operationToken, Operation: op})
 
@@ -386,6 +409,14 @@ func (p *expressionTokeniserImpl) handleToken(tokens []*token, index int, postPr
 
 	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)) {
+
+		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 {
@@ -395,18 +426,8 @@ func (p *expressionTokeniserImpl) handleToken(tokens []*token, index int, postPr
 	if index != len(tokens)-1 && currentToken.CheckForPostTraverse &&
 		tokens[index+1].TokenType == openCollect {
 
-		op := &Operation{OperationType: shortPipeOpType, Value: "PIPE"}
+		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

@@ -35,6 +35,7 @@ var addAssignOpType = &operationType{Type: "ADD_ASSIGN", NumArgs: 2, Precedence:
 
 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}
@@ -52,6 +53,7 @@ var shortPipeOpType = &operationType{Type: "SHORT_PIPE", NumArgs: 2, Precedence:
 var lengthOpType = &operationType{Type: "LENGTH", NumArgs: 0, Precedence: 50, Handler: lengthOperator}
 var collectOpType = &operationType{Type: "COLLECT", NumArgs: 0, Precedence: 50, Handler: collectOperator}
 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}
@@ -70,10 +72,10 @@ var splitStringOpType = &operationType{Type: "SPLIT", NumArgs: 1, Precedence: 50
 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}

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)
+	return d.GetMatchingNodes(context, assignmentOpNode)
 }
 
 func toNodes(candidate *CandidateNode) []*yaml.Node {
@@ -37,19 +36,24 @@ 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, expressionNode, add)
 }
 
-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)
 
-	target := lhs.CreateChild(nil, &yaml.Node{})
 	lhsNode := lhs.Node
 
+	if lhsNode.Tag == "!!null" {
+		return lhs.CreateChild(nil, rhs.Node), nil
+	}
+
+	target := lhs.CreateChild(nil, &yaml.Node{})
+
 	switch lhsNode.Kind {
 	case yaml.MappingNode:
 		return nil, fmt.Errorf("Maps not yet supported for addition")

pkg/yqlib/operator_add_test.go

@@ -5,6 +5,15 @@ import (
 )
 
 var addOperatorScenarios = []expressionScenario{
+	{
+		skipDoc:    true,
+		document:   `[{a: foo, b: bar}, {a: 1, b: 2}]`,
+		expression: ".[] | .a + .b",
+		expected: []string{
+			"D0, P[0 a], (!!str)::foobar\n",
+			"D0, P[1 a], (!!int)::3\n",
+		},
+	},
 	{
 		description: "Concatenate and assign arrays",
 		document:    `{a: {val: thing, b: [cat,dog]}}`,
@@ -103,6 +112,14 @@ var addOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::{a: 4}\n",
 		},
 	},
+	{
+		description:    "Add to null",
+		subdescription: "Adding to null simply returns the rhs",
+		expression:     `null + "cat"`,
+		expected: []string{
+			"D0, P[], (!!str)::cat\n",
+		},
+	},
 }
 
 func TestAddOperatorScenarios(t *testing.T) {

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, expressionNode, alternativeFunc)
 }
 
-func alternativeFunc(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+func alternativeFunc(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
 	lhs.Node = unwrapDoc(lhs.Node)
 	rhs.Node = unwrapDoc(rhs.Node)
 	log.Debugf("Alternative LHS: %v", lhs.Node.Tag)

pkg/yqlib/operator_anchors_aliases.go

@@ -6,146 +6,146 @@ 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, 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.SingleChildContext(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
 	}
-	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, 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.SingleChildContext(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
 			}
@@ -169,7 +169,7 @@ 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
 				}
@@ -178,14 +178,14 @@ func explodeNode(node *yaml.Node) error {
 					log.Debugf("an alias merge list!")
 					for index := 0; index < len(valueNode.Content); 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 +205,7 @@ 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 +221,15 @@ 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 +250,12 @@ 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_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, 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 +31,30 @@ 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) {
+	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.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 {
 			candidate.UpdateAttributesFrom(first.Value.(*CandidateNode))
 		}
 	}
-	return matchingNodes, nil
+	return context, nil
 }

pkg/yqlib/operator_booleans.go

@@ -25,8 +25,8 @@ func isTruthy(c *CandidateNode) (bool, error) {
 
 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) {
+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) {
 		lhs.Node = unwrapDoc(lhs.Node)
 		rhs.Node = unwrapDoc(rhs.Node)
 
@@ -44,35 +44,35 @@ func performBoolOp(op boolOp) func(d *dataTreeNavigator, lhs *CandidateNode, rhs
 	}
 }
 
-func orOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func orOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- orOp")
-	return crossFunction(d, matchingNodes, expressionNode, performBoolOp(
+	return crossFunction(d, context, expressionNode, performBoolOp(
 		func(b1 bool, b2 bool) bool {
 			return b1 || b2
 		}))
 }
 
-func andOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func andOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- AndOp")
-	return crossFunction(d, matchingNodes, expressionNode, performBoolOp(
+	return crossFunction(d, context, expressionNode, performBoolOp(
 		func(b1 bool, b2 bool) bool {
 			return b1 && b2
 		}))
 }
 
-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_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 +17,22 @@ import (
 ...
 */
 
-func collectObjectOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func collectObjectOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- collectObjectOperation")
 
-	if matchMap.Len() == 0 {
+	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 +41,59 @@ 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_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, 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.SingleChildContext(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_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))
@@ -67,7 +67,7 @@ func sequenceFor(d *dataTreeNavigator, matchingNode *CandidateNode, expressionNo
 	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

@@ -1,21 +1,20 @@
 package yqlib
 
 import (
-	"container/list"
 	"fmt"
 
 	yaml "gopkg.in/yaml.v3"
 )
 
-func deleteChildOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func deleteChildOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
-	nodesToDelete, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+	nodesToDelete, err := d.GetMatchingNodes(context, 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{
@@ -28,26 +27,26 @@ func deleteChildOperator(d *dataTreeNavigator, matchingNodes *list.List, express
 			Rhs:       createTraversalTree(candidate.Path[0:len(candidate.Path)-1], traversePreferences{}),
 		}
 
-		_, err := d.GetMatchingNodes(matchingNodes, deleteImmediateChildOpNode)
+		_, err := d.GetMatchingNodes(context, deleteImmediateChildOpNode)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 	}
-	return matchingNodes, nil
+	return context, nil
 }
 
-func deleteImmediateChildOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	parents, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+func deleteImmediateChildOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	parents, err := d.GetMatchingNodes(context, expressionNode.Rhs)
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
 	childPath := expressionNode.Operation.Value
 
 	log.Debug("childPath to remove %v", childPath)
 
-	for el := parents.Front(); el != nil; el = el.Next() {
+	for el := parents.MatchingNodes.Front(); el != nil; el = el.Next() {
 		parent := el.Value.(*CandidateNode)
 		parentNode := unwrapDoc(parent.Node)
 		if parentNode.Kind == yaml.MappingNode {
@@ -55,11 +54,11 @@ func deleteImmediateChildOperator(d *dataTreeNavigator, matchingNodes *list.List
 		} else if parentNode.Kind == yaml.SequenceNode {
 			deleteFromArray(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

@@ -37,6 +37,22 @@ 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",
+		},
+	},
 	{
 		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_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

@@ -1,15 +1,11 @@
 package yqlib
 
-import (
-	"container/list"
-)
-
-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, expressionNode, isEquals)
 }
 
-func isEquals(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
+func isEquals(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {
 	value := false
 
 	lhsNode := unwrapDoc(lhs.Node)

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_has.go

@@ -7,20 +7,20 @@ 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
+	rhs, err := d.GetMatchingNodes(context, expressionNode.Rhs)
+	wanted := rhs.MatchingNodes.Front().Value.(*CandidateNode).Node
 	wantedKey := wanted.Value
 
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
 
-	for el := matchingNodes.Front(); el != nil; el = el.Next() {
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
 		candidate := el.Value.(*CandidateNode)
 
 		// grab the first value
@@ -41,7 +41,7 @@ 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 +50,5 @@ func hasOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode
 			results.PushBack(createBooleanCandidate(candidate, false))
 		}
 	}
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_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

@@ -2,59 +2,25 @@ package yqlib
 
 import (
 	"fmt"
+	"strconv"
 
 	"container/list"
 
 	yaml "gopkg.in/yaml.v3"
 )
 
-type crossFunctionCalculation func(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error)
-
-func crossFunction(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode, calculation crossFunctionCalculation) (*list.List, error) {
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
-	if err != nil {
-		return nil, err
-	}
-	log.Debugf("crossFunction LHS len: %v", lhs.Len())
-
-	rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
-
-	if err != nil {
-		return nil, err
-	}
-	log.Debugf("crossFunction RHS len: %v", rhs.Len())
-
-	var results = list.New()
-
-	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
-}
-
 type multiplyPreferences struct {
 	AppendArrays  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)))
 }
 
-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)
@@ -64,25 +30,44 @@ 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)
 		}
 		return nil, fmt.Errorf("Cannot multiply %v with %v", lhs.Node.Tag, rhs.Node.Tag)
 	}
 }
 
-func mergeObjects(d *dataTreeNavigator, lhs *CandidateNode, rhs *CandidateNode, preferences multiplyPreferences) (*CandidateNode, error) {
+func multiplyIntegers(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 = "!!int"
+
+	lhsNum, err := strconv.Atoi(lhs.Node.Value)
+	if err != nil {
+		return nil, err
+	}
+	rhsNum, err := strconv.Atoi(rhs.Node.Value)
+	if err != nil {
+		return nil, err
+	}
+	target.Node.Value = fmt.Sprintf("%v", lhsNum*rhsNum)
+	return target, nil
+}
+
+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)
+	err := recursiveDecent(d, results, context.SingleChildContext(rhs), prefs)
 	if err != nil {
 		return nil, err
 	}
@@ -93,7 +78,7 @@ 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)
+		err := applyAssignment(d, context, pathIndexToStartFrom, lhs, el.Value.(*CandidateNode), preferences)
 		if err != nil {
 			return nil, err
 		}
@@ -101,7 +86,7 @@ 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))
 
@@ -118,7 +103,7 @@ func applyAssignment(d *dataTreeNavigator, pathIndexToStartFrom int, lhs *Candid
 
 	assignmentOpNode := &ExpressionNode{Operation: assignmentOp, Lhs: createTraversalTree(lhsPath, preferences.TraversePrefs), 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

@@ -13,6 +13,27 @@ var multiplyOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!map)::{a: {also: me}, b: {also: me}}\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `{a: 2, b: 5}`,
+		document2:  `{a: 3, b: 10}`,
+		expression: `.a * .b`,
+		expected: []string{
+			"D0, P[a], (!!int)::10\n",
+			"D0, P[a], (!!int)::20\n",
+			"D0, P[a], (!!int)::15\n",
+			"D0, P[a], (!!int)::30\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		document:   `{a: 2}`,
+		document2:  `{b: 10}`,
+		expression: `select(fi ==0) * select(fi==1)`,
+		expected: []string{
+			"D0, P[], (!!map)::{a: 2, b: 10}\n",
+		},
+	},
 	{
 		skipDoc:    true,
 		expression: `{} * {"cat":"dog"}`,
@@ -123,6 +144,14 @@ b:
 			"D0, P[a], (!!seq)::[{thing: two}]\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `{a: {array: [1]}, b: {}}`,
+		expression: `.b *+ .a`,
+		expected: []string{
+			"D0, P[b], (!!map)::{array: [1]}\n",
+		},
+	},
 	{
 		description: "Merge, appending arrays",
 		document:    `{a: {array: [1, 2, animal: dog], value: coconut}, b: {array: [3, 4, animal: cat], value: banana}}`,

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_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)
+		childContext := context.SingleChildContext(candidate)
+		childContext.DontAutoCreate = true
+		rhs, err := d.GetMatchingNodes(childContext, 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)
 			includeResult, errDecoding := isTruthy(result)
 			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 +14,14 @@ var selectOperatorScenarios = []expressionScenario{
 			"D0, P[1], (!!str)::goat\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.SingleChildContext(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_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

@@ -8,32 +8,32 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
-func joinStringOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func joinStringOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- joinStringOperator")
 	joinStr := ""
 
-	rhs, err := d.GetMatchingNodes(matchMap, expressionNode.Rhs)
+	rhs, err := d.GetMatchingNodes(context, expressionNode.Rhs)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-	if rhs.Front() != nil {
-		joinStr = rhs.Front().Value.(*CandidateNode).Node.Value
+	if rhs.MatchingNodes.Front() != nil {
+		joinStr = 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.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 +49,35 @@ 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, 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_style.go

@@ -26,43 +26,43 @@ func parseStyle(customStyle string) (yaml.Style, error) {
 	return 0, nil
 }
 
-func assignStyleOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func assignStyleOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
 	log.Debugf("AssignStyleOperator: %v")
 	var style yaml.Style
 	if !expressionNode.Operation.UpdateAssign {
-		rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context, expressionNode.Rhs)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
-		if rhs.Front() != nil {
-			style, err = parseStyle(rhs.Front().Value.(*CandidateNode).Node.Value)
+		if rhs.MatchingNodes.Front() != nil {
+			style, err = parseStyle(rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value)
 			if err != nil {
-				return nil, err
+				return Context{}, err
 			}
 		}
 	}
 
-	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 style of : %v", candidate.GetKey())
 		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
 			}
 
-			if rhs.Front() != nil {
-				style, err = parseStyle(rhs.Front().Value.(*CandidateNode).Node.Value)
+			if rhs.MatchingNodes.Front() != nil {
+				style, err = parseStyle(rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value)
 				if err != nil {
-					return nil, err
+					return Context{}, err
 				}
 			}
 		}
@@ -70,15 +70,15 @@ func assignStyleOperator(d *dataTreeNavigator, matchingNodes *list.List, express
 		candidate.Node.Style = style
 	}
 
-	return matchingNodes, nil
+	return context, nil
 }
 
-func getStyleOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getStyleOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("GetStyleOperator")
 
 	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)
 		var style string
 		switch candidate.Node.Style {
@@ -104,5 +104,5 @@ func getStyleOperator(d *dataTreeNavigator, matchingNodes *list.List, expression
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_tag.go

@@ -6,58 +6,58 @@ import (
 	yaml "gopkg.in/yaml.v3"
 )
 
-func assignTagOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func assignTagOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
 	log.Debugf("AssignTagOperator: %v")
 	tag := ""
 
 	if !expressionNode.Operation.UpdateAssign {
-		rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+		rhs, err := d.GetMatchingNodes(context, expressionNode.Rhs)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 
-		if rhs.Front() != nil {
-			tag = rhs.Front().Value.(*CandidateNode).Node.Value
+		if rhs.MatchingNodes.Front() != nil {
+			tag = 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 tag of : %v", candidate.GetKey())
 		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
 			}
 
-			if rhs.Front() != nil {
-				tag = rhs.Front().Value.(*CandidateNode).Node.Value
+			if rhs.MatchingNodes.Front() != nil {
+				tag = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Value
 			}
 		}
 		unwrapDoc(candidate.Node).Tag = tag
 	}
 
-	return matchingNodes, nil
+	return context, nil
 }
 
-func getTagOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func getTagOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("GetTagOperator")
 
 	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: unwrapDoc(candidate.Node).Tag, Tag: "!!str"}
 		result := candidate.CreateChild(nil, node)
 		results.PushBack(result)
 	}
 
-	return results, nil
+	return context.ChildContext(results), nil
 }

pkg/yqlib/operator_traverse_path.go

@@ -15,26 +15,26 @@ type traversePreferences struct {
 	DontAutoCreate  bool // by default, we automatically create entries on the fly.
 }
 
-func splat(d *dataTreeNavigator, matches *list.List, prefs traversePreferences) (*list.List, error) {
-	return traverseNodesWithArrayIndices(matches, make([]*yaml.Node, 0), prefs)
+func splat(d *dataTreeNavigator, context Context, prefs traversePreferences) (Context, error) {
+	return traverseNodesWithArrayIndices(context, make([]*yaml.Node, 0), prefs)
 }
 
-func traversePathOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	log.Debugf("-- Traversing")
-	var matchingNodeMap = list.New()
+func traversePathOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	log.Debugf("-- traversePathOperator")
+	var matches = list.New()
 
-	for el := matchMap.Front(); el != nil; el = el.Next() {
-		newNodes, err := traverse(d, el.Value.(*CandidateNode), expressionNode.Operation)
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		newNodes, err := traverse(d, context, el.Value.(*CandidateNode), expressionNode.Operation)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
-		matchingNodeMap.PushBackList(newNodes)
+		matches.PushBackList(newNodes)
 	}
 
-	return matchingNodeMap, nil
+	return context.ChildContext(matches), nil
 }
 
-func traverse(d *dataTreeNavigator, matchingNode *CandidateNode, operation *Operation) (*list.List, error) {
+func traverse(d *dataTreeNavigator, context Context, matchingNode *CandidateNode, operation *Operation) (*list.List, error) {
 	log.Debug("Traversing %v", NodeToString(matchingNode))
 	value := matchingNode.Node
 
@@ -55,7 +55,7 @@ func traverse(d *dataTreeNavigator, matchingNode *CandidateNode, operation *Oper
 	switch value.Kind {
 	case yaml.MappingNode:
 		log.Debug("its a map with %v entries", len(value.Content)/2)
-		return traverseMap(matchingNode, operation.StringValue, operation.Preferences.(traversePreferences), false)
+		return traverseMap(context, matchingNode, operation.StringValue, operation.Preferences.(traversePreferences), false)
 
 	case yaml.SequenceNode:
 		log.Debug("its a sequence of %v things!", len(value.Content))
@@ -64,43 +64,59 @@ func traverse(d *dataTreeNavigator, matchingNode *CandidateNode, operation *Oper
 	case yaml.AliasNode:
 		log.Debug("its an alias!")
 		matchingNode.Node = matchingNode.Node.Alias
-		return traverse(d, matchingNode, operation)
+		return traverse(d, context, matchingNode, operation)
 	case yaml.DocumentNode:
 		log.Debug("digging into doc node")
 
-		return traverse(d, matchingNode.CreateChild(nil, matchingNode.Node.Content[0]), operation)
+		return traverse(d, context, matchingNode.CreateChild(nil, matchingNode.Node.Content[0]), operation)
 	default:
 		return list.New(), nil
 	}
 }
 
-func traverseArrayOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	// rhs is a collect expression that will yield indexes to retreive of the arrays
+func traverseArrayOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 
-	rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+	//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
 	}
 
-	var indicesToTraverse = rhs.Front().Value.(*CandidateNode).Node.Content
-	return traverseNodesWithArrayIndices(matchingNodes, indicesToTraverse, traversePreferences{})
+	// rhs is a collect expression that will yield indexes to retreive of the arrays
+
+	rhs, err := d.GetMatchingNodes(context, expressionNode.Rhs)
+	if err != nil {
+		return Context{}, err
+	}
+
+	var indicesToTraverse = rhs.MatchingNodes.Front().Value.(*CandidateNode).Node.Content
+
+	//now we traverse the result of the lhs against the indices we found
+	result, err := traverseNodesWithArrayIndices(lhs, indicesToTraverse, traversePreferences{})
+	if err != nil {
+		return Context{}, err
+	}
+	return context.ChildContext(result.MatchingNodes), nil
 }
 
-func traverseNodesWithArrayIndices(matchingNodes *list.List, indicesToTraverse []*yaml.Node, prefs traversePreferences) (*list.List, error) {
+func traverseNodesWithArrayIndices(context Context, indicesToTraverse []*yaml.Node, prefs traversePreferences) (Context, error) {
 	var matchingNodeMap = 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)
-		newNodes, err := traverseArrayIndices(candidate, indicesToTraverse, prefs)
+		newNodes, err := traverseArrayIndices(context, candidate, indicesToTraverse, prefs)
 		if err != nil {
-			return nil, err
+			return Context{}, err
 		}
 		matchingNodeMap.PushBackList(newNodes)
 	}
 
-	return matchingNodeMap, nil
+	return context.ChildContext(matchingNodeMap), nil
 }
 
-func traverseArrayIndices(matchingNode *CandidateNode, indicesToTraverse []*yaml.Node, prefs traversePreferences) (*list.List, error) { // call this if doc / alias like the other traverse
+func traverseArrayIndices(context Context, matchingNode *CandidateNode, indicesToTraverse []*yaml.Node, prefs traversePreferences) (*list.List, error) { // call this if doc / alias like the other traverse
 	node := matchingNode.Node
 	if node.Tag == "!!null" {
 		log.Debugf("OperatorArrayTraverse got a null - turning it into an empty array")
@@ -111,28 +127,28 @@ func traverseArrayIndices(matchingNode *CandidateNode, indicesToTraverse []*yaml
 
 	if node.Kind == yaml.AliasNode {
 		matchingNode.Node = node.Alias
-		return traverseArrayIndices(matchingNode, indicesToTraverse, prefs)
+		return traverseArrayIndices(context, matchingNode, indicesToTraverse, prefs)
 	} else if node.Kind == yaml.SequenceNode {
 		return traverseArrayWithIndices(matchingNode, indicesToTraverse)
 	} else if node.Kind == yaml.MappingNode {
-		return traverseMapWithIndices(matchingNode, indicesToTraverse, prefs)
+		return traverseMapWithIndices(context, matchingNode, indicesToTraverse, prefs)
 	} else if node.Kind == yaml.DocumentNode {
-		return traverseArrayIndices(matchingNode.CreateChild(nil, matchingNode.Node.Content[0]), indicesToTraverse, prefs)
+		return traverseArrayIndices(context, matchingNode.CreateChild(nil, matchingNode.Node.Content[0]), indicesToTraverse, prefs)
 	}
 	log.Debugf("OperatorArrayTraverse skipping %v as its a %v", matchingNode, node.Tag)
 	return list.New(), nil
 }
 
-func traverseMapWithIndices(candidate *CandidateNode, indices []*yaml.Node, prefs traversePreferences) (*list.List, error) {
+func traverseMapWithIndices(context Context, candidate *CandidateNode, indices []*yaml.Node, prefs traversePreferences) (*list.List, error) {
 	if len(indices) == 0 {
-		return traverseMap(candidate, "", prefs, true)
+		return traverseMap(context, candidate, "", prefs, true)
 	}
 
 	var matchingNodeMap = list.New()
 
 	for _, indexNode := range indices {
 		log.Debug("traverseMapWithIndices: %v", indexNode.Value)
-		newNodes, err := traverseMap(candidate, indexNode.Value, prefs, false)
+		newNodes, err := traverseMap(context, candidate, indexNode.Value, prefs, false)
 		if err != nil {
 			return nil, err
 		}
@@ -187,7 +203,7 @@ func keyMatches(key *yaml.Node, wantedKey string) bool {
 	return matchKey(key.Value, wantedKey)
 }
 
-func traverseMap(matchingNode *CandidateNode, key string, prefs traversePreferences, splat bool) (*list.List, error) {
+func traverseMap(context Context, matchingNode *CandidateNode, key string, prefs traversePreferences, splat bool) (*list.List, error) {
 	var newMatches = orderedmap.NewOrderedMap()
 	err := doTraverseMap(newMatches, matchingNode, key, prefs, splat)
 
@@ -195,7 +211,7 @@ func traverseMap(matchingNode *CandidateNode, key string, prefs traversePreferen
 		return nil, err
 	}
 
-	if !prefs.DontAutoCreate && newMatches.Len() == 0 {
+	if !prefs.DontAutoCreate && !context.DontAutoCreate && newMatches.Len() == 0 {
 		//no matches, create one automagically
 		valueNode := &yaml.Node{Tag: "!!null", Kind: yaml.ScalarNode, Value: "null"}
 		node := matchingNode.Node

pkg/yqlib/operator_traverse_path_test.go

@@ -63,6 +63,14 @@ var traversePathOperatorScenarios = []expressionScenario{
 			"D0, P[apple], (!!str)::crispy yum\n",
 		},
 	},
+	{
+		skipDoc:    true,
+		document:   `{b: apple, fruit: {apple: yum, banana: smooth}}`,
+		expression: `.fruit[.b]`,
+		expected: []string{
+			"D0, P[fruit apple], (!!str)::yum\n",
+		},
+	},
 	{
 		description:    "Children don't exist",
 		subdescription: "Nodes are added dynamically while traversing",
@@ -83,7 +91,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   `{}`,
-		expression: `.a.[1]`,
+		expression: `.a[1]`,
 		expected: []string{
 			"D0, P[a 1], (!!null)::null\n",
 		},
@@ -154,7 +162,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   `{a: &cat {c: frog}, b: *cat}`,
-		expression: `.b.[]`,
+		expression: `.b[]`,
 		expected: []string{
 			"D0, P[b c], (!!str)::frog\n",
 		},
@@ -236,7 +244,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   mergeDocSample,
-		expression: `.foobar.[]`,
+		expression: `.foobar[]`,
 		expected: []string{
 			"D0, P[foobar c], (!!str)::foo_c\n",
 			"D0, P[foobar a], (!!str)::foo_a\n",
@@ -298,7 +306,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   mergeDocSample,
-		expression: `.foobarList.[]`,
+		expression: `.foobarList[]`,
 		expected: []string{
 			"D0, P[foobarList b], (!!str)::bar_b\n",
 			"D0, P[foobarList a], (!!str)::foo_a\n",
@@ -344,7 +352,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   `{a: [a,b,c]}`,
-		expression: `.a.[0, 2]`,
+		expression: `.a[0, 2]`,
 		expected: []string{
 			"D0, P[a 0], (!!str)::a\n",
 			"D0, P[a 2], (!!str)::c\n",
@@ -361,7 +369,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   `{a: [a,b,c]}`,
-		expression: `.a.[-1]`,
+		expression: `.a[-1]`,
 		expected: []string{
 			"D0, P[a -1], (!!str)::c\n",
 		},
@@ -377,7 +385,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   `{a: [a,b,c]}`,
-		expression: `.a.[-2]`,
+		expression: `.a[-2]`,
 		expected: []string{
 			"D0, P[a -2], (!!str)::b\n",
 		},
@@ -395,7 +403,7 @@ var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
 		document:   `{a: [a,b,c]}`,
-		expression: `.a.[]`,
+		expression: `.a[]`,
 		expected: []string{
 			"D0, P[a 0], (!!str)::a\n",
 			"D0, P[a 1], (!!str)::b\n",

pkg/yqlib/operator_union.go

@@ -1,19 +1,17 @@
 package yqlib
 
-import "container/list"
-
-func unionOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	lhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Lhs)
+func unionOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-	rhs, err := d.GetMatchingNodes(matchingNodes, expressionNode.Rhs)
+	rhs, err := d.GetMatchingNodes(context, expressionNode.Rhs)
 	if err != nil {
-		return nil, err
+		return Context{}, err
 	}
-	for el := rhs.Front(); el != nil; el = el.Next() {
+	for el := rhs.MatchingNodes.Front(); el != nil; el = el.Next() {
 		node := el.Value.(*CandidateNode)
-		lhs.PushBack(node)
+		lhs.MatchingNodes.PushBack(node)
 	}
 	return lhs, nil
 }

pkg/yqlib/operator_value.go

@@ -1,8 +1,6 @@
 package yqlib
 
-import "container/list"
-
-func valueOperator(d *dataTreeNavigator, matchMap *list.List, expressionNode *ExpressionNode) (*list.List, error) {
+func valueOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debug("value = %v", expressionNode.Operation.CandidateNode.Node.Value)
-	return nodeToMap(expressionNode.Operation.CandidateNode), nil
+	return context.SingleChildContext(expressionNode.Operation.CandidateNode), nil
 }

pkg/yqlib/operator_variables.go

@@ -0,0 +1,29 @@
+package yqlib
+
+import (
+	"container/list"
+	"fmt"
+)
+
+func getVariableOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	variableName := expressionNode.Operation.StringValue
+	log.Debug("getVariableOperator %v", variableName)
+	result := context.GetVariable(variableName)
+	if result == nil {
+		result = list.New()
+	}
+	return context.ChildContext(result), nil
+}
+
+func assignVariableOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
+	if err != nil {
+		return Context{}, nil
+	}
+	if expressionNode.Rhs.Operation.OperationType.Type != "GET_VARIABLE" {
+		return Context{}, fmt.Errorf("RHS of 'as' operator must be a variable name e.g. $foo")
+	}
+	variableName := expressionNode.Rhs.Operation.StringValue
+	context.SetVariable(variableName, lhs.MatchingNodes)
+	return context, nil
+}

pkg/yqlib/operator_variables_test.go

@@ -0,0 +1,45 @@
+package yqlib
+
+import (
+	"testing"
+)
+
+var variableOperatorScenarios = []expressionScenario{
+	{
+		description: "Single value variable",
+		document:    `a: cat`,
+		expression:  `.a as $foo | $foo`,
+		expected: []string{
+			"D0, P[a], (!!str)::cat\n",
+		},
+	},
+	{
+		description: "Multi value variable",
+		document:    `[cat, dog]`,
+		expression:  `.[] as $foo | $foo`,
+		expected: []string{
+			"D0, P[0], (!!str)::cat\n",
+			"D0, P[1], (!!str)::dog\n",
+		},
+	},
+	{
+		description:    "Using variables as a lookup",
+		subdescription: "Example taken from [jq](https://stedolan.github.io/jq/manual/#Variable/SymbolicBindingOperator:...as$identifier|...)",
+		document: `{"posts": [{"title": "Frist psot", "author": "anon"},
+		{"title": "A well-written article", "author": "person1"}],
+"realnames": {"anon": "Anonymous Coward",
+				"person1": "Person McPherson"}}`,
+		expression: `.realnames as $names | .posts[] | {"title":.title, "author": $names[.author]}`,
+		expected: []string{
+			"D0, P[], (!!map)::title: \"Frist psot\"\nauthor: \"Anonymous Coward\"\n",
+			"D0, P[], (!!map)::title: \"A well-written article\"\nauthor: \"Person McPherson\"\n",
+		},
+	},
+}
+
+func TestVariableOperatorScenarios(t *testing.T) {
+	for _, tt := range variableOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	documentScenarios(t, "Variable Operators", variableOperatorScenarios)
+}

pkg/yqlib/operators.go

@@ -7,7 +7,7 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
-type operatorHandler func(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error)
+type operatorHandler func(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error)
 
 func unwrapDoc(node *yaml.Node) *yaml.Node {
 	if node.Kind == yaml.DocumentNode {
@@ -16,8 +16,67 @@ func unwrapDoc(node *yaml.Node) *yaml.Node {
 	return node
 }
 
-func emptyOperator(d *dataTreeNavigator, matchingNodes *list.List, expressionNode *ExpressionNode) (*list.List, error) {
-	return list.New(), nil
+func emptyOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	context.MatchingNodes = list.New()
+	return context, nil
+}
+
+type crossFunctionCalculation func(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error)
+
+func doCrossFunc(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode, calculation crossFunctionCalculation) (Context, error) {
+	var results = list.New()
+	lhs, err := d.GetMatchingNodes(context, expressionNode.Lhs)
+	if err != nil {
+		return Context{}, err
+	}
+	log.Debugf("crossFunction LHS len: %v", lhs.MatchingNodes.Len())
+
+	rhs, err := d.GetMatchingNodes(context, expressionNode.Rhs)
+
+	if err != nil {
+		return Context{}, err
+	}
+
+	for el := lhs.MatchingNodes.Front(); el != nil; el = el.Next() {
+		lhsCandidate := el.Value.(*CandidateNode)
+
+		for rightEl := rhs.MatchingNodes.Front(); rightEl != nil; rightEl = rightEl.Next() {
+			log.Debugf("Applying calc")
+			rhsCandidate := rightEl.Value.(*CandidateNode)
+			resultCandidate, err := calculation(d, context, lhsCandidate, rhsCandidate)
+			if err != nil {
+				return Context{}, err
+			}
+			results.PushBack(resultCandidate)
+		}
+
+	}
+	return context.ChildContext(results), nil
+}
+
+func crossFunction(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode, calculation crossFunctionCalculation) (Context, error) {
+	var results = list.New()
+
+	var evaluateAllTogether = true
+	for matchEl := context.MatchingNodes.Front(); matchEl != nil; matchEl = matchEl.Next() {
+		evaluateAllTogether = evaluateAllTogether && matchEl.Value.(*CandidateNode).EvaluateTogether
+		if !evaluateAllTogether {
+			break
+		}
+	}
+	if evaluateAllTogether {
+		return doCrossFunc(d, context, expressionNode, calculation)
+	}
+
+	for matchEl := context.MatchingNodes.Front(); matchEl != nil; matchEl = matchEl.Next() {
+		innerResults, err := doCrossFunc(d, context.SingleChildContext(matchEl.Value.(*CandidateNode)), expressionNode, calculation)
+		if err != nil {
+			return Context{}, err
+		}
+		results.PushBackList(innerResults.MatchingNodes)
+	}
+
+	return context.ChildContext(results), nil
 }
 
 func createBooleanCandidate(owner *CandidateNode, value bool) *CandidateNode {
@@ -29,12 +88,6 @@ func createBooleanCandidate(owner *CandidateNode, value bool) *CandidateNode {
 	return owner.CreateChild(nil, node)
 }
 
-func nodeToMap(candidate *CandidateNode) *list.List {
-	elMap := list.New()
-	elMap.PushBack(candidate)
-	return elMap
-}
-
 func createTraversalTree(path []interface{}, traversePrefs traversePreferences) *ExpressionNode {
 	if len(path) == 0 {
 		return &ExpressionNode{Operation: &Operation{OperationType: selfReferenceOpType}}

pkg/yqlib/operators_test.go

@@ -27,7 +27,6 @@ type expressionScenario struct {
 }
 
 func testScenario(t *testing.T, s *expressionScenario) {
-	var results *list.List
 	var err error
 
 	node, err := NewExpressionParser().ParseExpression(s.expression)
@@ -66,13 +65,13 @@ func testScenario(t *testing.T, s *expressionScenario) {
 		os.Setenv("myenv", s.environmentVariable)
 	}
 
-	results, err = NewDataTreeNavigator().GetMatchingNodes(inputs, node)
+	context, err := NewDataTreeNavigator().GetMatchingNodes(Context{MatchingNodes: inputs}, node)
 
 	if err != nil {
 		t.Error(fmt.Errorf("%v: %v", err, s.expression))
 		return
 	}
-	test.AssertResultComplexWithContext(t, s.expected, resultsToString(results), fmt.Sprintf("exp: %v\ndoc: %v", s.expression, s.document))
+	test.AssertResultComplexWithContext(t, s.expected, resultsToString(context.MatchingNodes), fmt.Sprintf("exp: %v\ndoc: %v", s.expression, s.document))
 }
 
 func resultsToString(results *list.List) []string {
@@ -252,12 +251,12 @@ func documentOutput(t *testing.T, w *bufio.Writer, s expressionScenario, formatt
 
 	}
 
-	results, err := NewDataTreeNavigator().GetMatchingNodes(inputs, node)
+	context, err := NewDataTreeNavigator().GetMatchingNodes(Context{MatchingNodes: inputs}, node)
 	if err != nil {
 		t.Error(err, s.expression)
 	}
 
-	err = printer.PrintResults(results)
+	err = printer.PrintResults(context.MatchingNodes)
 	if err != nil {
 		t.Error(err, s.expression)
 	}

pkg/yqlib/printer.go

@@ -74,14 +74,14 @@ func (p *resultsPrinter) safelyFlush(writer *bufio.Writer) {
 
 func (p *resultsPrinter) PrintResults(matchingNodes *list.List) error {
 	log.Debug("PrintResults for %v matches", matchingNodes.Len())
-	var err error
 	if p.outputToJSON {
 		explodeOp := Operation{OperationType: explodeOpType}
 		explodeNode := ExpressionNode{Operation: &explodeOp}
-		matchingNodes, err = p.treeNavigator.GetMatchingNodes(matchingNodes, &explodeNode)
+		context, err := p.treeNavigator.GetMatchingNodes(Context{MatchingNodes: matchingNodes}, &explodeNode)
 		if err != nil {
 			return err
 		}
+		matchingNodes = context.MatchingNodes
 	}
 
 	bufferedWriter := bufio.NewWriter(p.writer)

pkg/yqlib/printer_test.go

@@ -3,6 +3,7 @@ package yqlib
 import (
 	"bufio"
 	"bytes"
+	"container/list"
 	"strings"
 	"testing"
 
@@ -16,6 +17,12 @@ a: apple
 a: coconut
 `
 
+func nodeToList(candidate *CandidateNode) *list.List {
+	elMap := list.New()
+	elMap.PushBack(candidate)
+	return elMap
+}
+
 func TestPrinterMultipleDocsInSequence(t *testing.T) {
 	var output bytes.Buffer
 	var writer = bufio.NewWriter(&output)
@@ -27,13 +34,13 @@ func TestPrinterMultipleDocsInSequence(t *testing.T) {
 	}
 
 	el := inputs.Front()
-	sample1 := nodeToMap(el.Value.(*CandidateNode))
+	sample1 := nodeToList(el.Value.(*CandidateNode))
 
 	el = el.Next()
-	sample2 := nodeToMap(el.Value.(*CandidateNode))
+	sample2 := nodeToList(el.Value.(*CandidateNode))
 
 	el = el.Next()
-	sample3 := nodeToMap(el.Value.(*CandidateNode))
+	sample3 := nodeToList(el.Value.(*CandidateNode))
 
 	err = printer.PrintResults(sample1)
 	if err != nil {
@@ -68,19 +75,19 @@ func TestPrinterMultipleFilesInSequence(t *testing.T) {
 	elNode := el.Value.(*CandidateNode)
 	elNode.Document = 0
 	elNode.FileIndex = 0
-	sample1 := nodeToMap(elNode)
+	sample1 := nodeToList(elNode)
 
 	el = el.Next()
 	elNode = el.Value.(*CandidateNode)
 	elNode.Document = 0
 	elNode.FileIndex = 1
-	sample2 := nodeToMap(elNode)
+	sample2 := nodeToList(elNode)
 
 	el = el.Next()
 	elNode = el.Value.(*CandidateNode)
 	elNode.Document = 0
 	elNode.FileIndex = 2
-	sample3 := nodeToMap(elNode)
+	sample3 := nodeToList(elNode)
 
 	err = printer.PrintResults(sample1)
 	if err != nil {

pkg/yqlib/stream_evaluator.go

@@ -41,11 +41,11 @@ func (s *streamEvaluator) EvaluateNew(expression string, printer Printer) error
 	inputList := list.New()
 	inputList.PushBack(candidateNode)
 
-	matches, errorParsing := s.treeNavigator.GetMatchingNodes(inputList, node)
+	result, errorParsing := s.treeNavigator.GetMatchingNodes(Context{MatchingNodes: inputList}, node)
 	if errorParsing != nil {
 		return errorParsing
 	}
-	return printer.PrintResults(matches)
+	return printer.PrintResults(result.MatchingNodes)
 }
 
 func (s *streamEvaluator) EvaluateFiles(expression string, filenames []string, printer Printer) error {
@@ -97,11 +97,11 @@ func (s *streamEvaluator) Evaluate(filename string, reader io.Reader, node *Expr
 		inputList := list.New()
 		inputList.PushBack(candidateNode)
 
-		matches, errorParsing := s.treeNavigator.GetMatchingNodes(inputList, node)
+		result, errorParsing := s.treeNavigator.GetMatchingNodes(Context{MatchingNodes: inputList}, node)
 		if errorParsing != nil {
 			return errorParsing
 		}
-		err := printer.PrintResults(matches)
+		err := printer.PrintResults(result.MatchingNodes)
 		if err != nil {
 			return err
 		}

pkg/yqlib/utils.go

@@ -36,10 +36,11 @@ func readDocuments(reader io.Reader, filename string, fileIndex int) (*list.List
 			return nil, errorReading
 		}
 		candidateNode := &CandidateNode{
-			Document:  currentIndex,
-			Filename:  filename,
-			Node:      &dataBucket,
-			FileIndex: fileIndex,
+			Document:         currentIndex,
+			Filename:         filename,
+			Node:             &dataBucket,
+			FileIndex:        fileIndex,
+			EvaluateTogether: true,
 		}
 
 		inputList.PushBack(candidateNode)

snap/snapcraft.yaml

@@ -1,5 +1,5 @@
 name: yq
-version: '4.4.0'
+version: '4.5.0'
 summary: A lightweight and portable command-line YAML processor
 description: |
   The aim of the project is to be the jq or sed of yaml files.