Updated var to work like jq #934

pkg/yqlib/context.go

@@ -61,9 +61,31 @@ func (n *Context) ToString() string {
 	return result + NodesToString(n.MatchingNodes)
 }
 
+func (n *Context) DeepClone() Context {
+	clone := Context{}
+	err := copier.Copy(&clone, n)
+	// copier doesn't do lists properly for some reason
+	clone.MatchingNodes = list.New()
+	for el := n.MatchingNodes.Front(); el != nil; el = el.Next() {
+		clonedNode, err := el.Value.(*CandidateNode).Copy()
+		if err != nil {
+			log.Error("Error cloning context :(")
+			panic(err)
+		}
+		clone.MatchingNodes.PushBack(clonedNode)
+	}
+
+	if err != nil {
+		log.Error("Error cloning context :(")
+		panic(err)
+	}
+	return clone
+}
+
 func (n *Context) Clone() Context {
 	clone := Context{}
 	err := copier.Copy(&clone, n)
+
 	if err != nil {
 		log.Error("Error cloning context :(")
 		panic(err)

pkg/yqlib/doc/Style.md

@@ -18,7 +18,7 @@ a:
 ```
 
 ## Update and set style of a particular node using path variables
-You can use a variable to re-use a path
+You can use a variable reference to re-use a path
 
 Given a sample.yml file of:
 ```yaml
@@ -28,7 +28,7 @@ a:
 ```
 then
 ```bash
-yq eval '.a.b as $x | $x = "new" | $x style="double"' sample.yml
+yq eval '.a.b ref $x | $x = "new" | $x style="double"' sample.yml
 ```
 will output
 ```yaml

pkg/yqlib/doc/Variable Operators.md

@@ -1,4 +1,6 @@
-For more complex scenarios, variables can be used to hold values of expression to be used in other expressions.
+Like the `jq` equivalents, variables are sometimes required for the more complex expressions (or swapping values between fields).
+
+Note that there is also an additional `ref` operator that holds a reference (instead of a copy) of the path, allowing you to make multiple changes to the same path.
 
 ## Single value variable
 Given a sample.yml file of:
@@ -56,3 +58,37 @@ title: A well-written article
 author: Person McPherson
 ```
 
+## Using variables to swap values
+Given a sample.yml file of:
+```yaml
+a: a_value
+b: b_value
+```
+then
+```bash
+yq eval '.a as $x  | .b as $y | .b = $x | .a = $y' sample.yml
+```
+will output
+```yaml
+a: b_value
+b: a_value
+```
+
+## Use ref to reference a path repeatedly
+Given a sample.yml file of:
+```yaml
+a:
+  b: thing
+  c: something
+```
+then
+```bash
+yq eval '.a.b ref $x | $x = "new" | $x style="double"' sample.yml
+```
+will output
+```yaml
+a:
+  b: "new"
+  c: something
+```
+

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

@@ -1 +1,3 @@
-For more complex scenarios, variables can be used to hold values of expression to be used in other expressions.
+Like the `jq` equivalents, variables are sometimes required for the more complex expressions (or swapping values between fields).
+
+Note that there is also an additional `ref` operator that holds a reference (instead of a copy) of the path, allowing you to make multiple changes to the same path.

pkg/yqlib/expression_tokeniser.go

@@ -367,7 +367,8 @@ func initLexer() (*lex.Lexer, error) {
 	lexer.Add([]byte(`\-`), opToken(subtractOpType))
 	lexer.Add([]byte(`\-=`), opToken(subtractAssignOpType))
 	lexer.Add([]byte(`\$[a-zA-Z_-0-9]+`), getVariableOpToken())
-	lexer.Add([]byte(`as`), opToken(assignVariableOpType))
+	lexer.Add([]byte(`as`), opTokenWithPrefs(assignVariableOpType, nil, assignVarPreferences{}))
+	lexer.Add([]byte(`ref`), opTokenWithPrefs(assignVariableOpType, nil, assignVarPreferences{IsReference: true}))
 
 	err := lexer.CompileNFA()
 	if err != nil {

pkg/yqlib/operator_collect_object.go

@@ -20,8 +20,7 @@ import (
 func collectObjectOperator(d *dataTreeNavigator, originalContext Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- collectObjectOperation")
 
-	context := originalContext.Clone()
-	context.DontAutoCreate = false
+	context := originalContext.WritableClone()
 
 	if context.MatchingNodes.Len() == 0 {
 		node := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map", Value: "{}"}

pkg/yqlib/operator_style_test.go

@@ -15,9 +15,9 @@ var styleOperatorScenarios = []expressionScenario{
 	},
 	{
 		description:    "Update and set style of a particular node using path variables",
-		subdescription: "You can use a variable to re-use a path",
+		subdescription: "You can use a variable reference to re-use a path",
 		document:       `a: {b: thing, c: something}`,
-		expression:     `.a.b as $x | $x = "new" | $x style="double"`,
+		expression:     `.a.b ref $x | $x = "new" | $x style="double"`,
 		expected: []string{
 			"D0, P[], (doc)::a: {b: \"new\", c: something}\n",
 		},

pkg/yqlib/operator_variables.go

@@ -15,6 +15,10 @@ func getVariableOperator(d *dataTreeNavigator, context Context, expressionNode *
 	return context.ChildContext(result), nil
 }
 
+type assignVarPreferences struct {
+	IsReference bool
+}
+
 func assignVariableOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	lhs, err := d.GetMatchingNodes(context.ReadOnlyClone(), expressionNode.Lhs)
 	if err != nil {
@@ -24,6 +28,15 @@ func assignVariableOperator(d *dataTreeNavigator, context Context, expressionNod
 		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)
+
+	prefs := expressionNode.Operation.Preferences.(assignVarPreferences)
+
+	var variableValue *list.List
+	if prefs.IsReference {
+		variableValue = lhs.MatchingNodes
+	} else {
+		variableValue = lhs.DeepClone().MatchingNodes
+	}
+	context.SetVariable(variableName, variableValue)
 	return context, nil
 }

pkg/yqlib/operator_variables_test.go

@@ -43,6 +43,22 @@ var variableOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!map)::title: \"A well-written article\"\nauthor: \"Person McPherson\"\n",
 		},
 	},
+	{
+		description: "Using variables to swap values",
+		document:    "a: a_value\nb: b_value",
+		expression:  `.a as $x  | .b as $y | .b = $x | .a = $y`,
+		expected: []string{
+			"D0, P[], (doc)::a: b_value\nb: a_value\n",
+		},
+	},
+	{
+		description: "Use ref to reference a path repeatedly",
+		document:    `a: {b: thing, c: something}`,
+		expression:  `.a.b ref $x | $x = "new" | $x style="double"`,
+		expected: []string{
+			"D0, P[], (doc)::a: {b: \"new\", c: something}\n",
+		},
+	},
 }
 
 func TestVariableOperatorScenarios(t *testing.T) {