Added context variable for reduce

pkg/yqlib/doc/File Operators.md

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

pkg/yqlib/doc/Reduce.md

@@ -1,11 +1,27 @@
 Reduce is a powerful way to process a collection of data into a new form.
 
+```
+<exp> as $<name> ireduce (<init>; <block>)
+```
+
+e.g.
+
+```
+.[] as $item ireduce (0; . + $item)
+```
+
+On the LHS we are configuring the collection of items that will be reduced `<exp>` as well as what each element will be called `$<name>`. Note that the array has been splatted into its individual elements.
+
+On the RHS there is `<init>`, the starting value of the accumulator and `<block>`, the expression that will update the accumulator for each element in the collection. 
+
+Note that within the block expression, `.` will evaluate to the current value of the accumulator. This effectively means that within the `reduce` block you can no longer access data other than elements of the array set as `$<name>`. For simple things, this is probably fine, but often you will need to refer to other data elements.
+
+This can be done by setting a variable using `as` and piping that into the `reduce` operation, or you can simply refer to `$context` which is exactly that, automatically set for you for convenience. See examples below.
+
 ## yq vs jq syntax
-Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn't as sophisticated as `jq` and its only supports infix notation (e.g. a + b, the operator is in the middle of the two parameters) - where as `jq` uses a mix of infix notation with _prefix_ notation (e.g. `reduce a b` is like writing `+ a b`).
+Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn't as sophisticated as `jq` and its only supports infix notation (e.g. a + b, where the operator is in the middle of the two parameters) - where as `jq` uses a mix of infix notation with _prefix_ notation (e.g. `reduce a b` is like writing `+ a b`).
 
 To that end, the reduce operator is called `ireduce` for backwards compatability if a prefix version of `reduce` is ever added.
-
-
 ## Sum numbers
 Given a sample.yml file of:
 ```yaml
@@ -41,3 +57,47 @@ Cathy: apples
 Bob: bananas
 ```
 
+## Merge all documents together - using context
+The _$context_ variable set by reduce lets you access the data outside the reduce block.
+
+Given a sample.yml file of:
+```yaml
+a: cat
+```
+And another sample another.yml file of:
+```yaml
+b: dog
+```
+then
+```bash
+yq eval-all 'fileIndex as $item ireduce ({}; . * ($context | select(fileIndex==$item)) )' sample.yml another.yml
+```
+will output
+```yaml
+a: cat
+b: dog
+```
+
+## Merge all documents together - without using context
+`$context` is just a convenient variable that `reduce` sets, you can use your own for more control
+
+Given a sample.yml file of:
+```yaml
+c:
+  a: cat
+```
+And another sample another.yml file of:
+```yaml
+c:
+  b: dog
+```
+then
+```bash
+yq eval-all '.c as $root | fileIndex as $item ireduce ({}; . * ($root | select(fileIndex==$item)) )' sample.yml another.yml
+```
+will output
+```yaml
+a: cat
+b: dog
+```
+

pkg/yqlib/doc/headers/Reduce.md

@@ -1,7 +1,24 @@
 Reduce is a powerful way to process a collection of data into a new form.
 
+```
+<exp> as $<name> ireduce (<init>; <block>)
+```
+
+e.g.
+
+```
+.[] as $item ireduce (0; . + $item)
+```
+
+On the LHS we are configuring the collection of items that will be reduced `<exp>` as well as what each element will be called `$<name>`. Note that the array has been splatted into its individual elements.
+
+On the RHS there is `<init>`, the starting value of the accumulator and `<block>`, the expression that will update the accumulator for each element in the collection. 
+
+Note that within the block expression, `.` will evaluate to the current value of the accumulator. This effectively means that within the `reduce` block you can no longer access data other than elements of the array set as `$<name>`. For simple things, this is probably fine, but often you will need to refer to other data elements.
+
+This can be done by setting a variable using `as` and piping that into the `reduce` operation, or you can simply refer to `$context` which is exactly that, automatically set for you for convenience. See examples below.
+
 ## yq vs jq syntax
-Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn't as sophisticated as `jq` and its only supports infix notation (e.g. a + b, the operator is in the middle of the two parameters) - where as `jq` uses a mix of infix notation with _prefix_ notation (e.g. `reduce a b` is like writing `+ a b`).
-
-To that end, the reduce operator is called `ireduce` for backwards compatability if a prefix version of `reduce` is ever added.
+Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn't as sophisticated as `jq` and its only supports infix notation (e.g. a + b, where the operator is in the middle of the two parameters) - where as `jq` uses a mix of infix notation with _prefix_ notation (e.g. `reduce a b` is like writing `+ a b`).
 
+To that end, the reduce operator is called `ireduce` for backwards compatability if a prefix version of `reduce` is ever added.

pkg/yqlib/expression_tokeniser.go

@@ -186,7 +186,7 @@ func getVariableOpToken() lex.Action {
 	return func(s *lex.Scanner, m *machines.Match) (interface{}, error) {
 		value := string(m.Bytes)
 
-		value = value[1 : len(value)-1]
+		value = value[1:]
 
 		getVarOperation := createValueOperation(value, value)
 		getVarOperation.OperationType = getVariableOpType

pkg/yqlib/lib.go

@@ -25,7 +25,7 @@ type operationType struct {
 
 var orOpType = &operationType{Type: "OR", NumArgs: 2, Precedence: 20, Handler: orOperator}
 var andOpType = &operationType{Type: "AND", NumArgs: 2, Precedence: 20, Handler: andOperator}
-var reduceOpType = &operationType{Type: "REDUCE", NumArgs: 2, Precedence: 5, Handler: reduceOperator}
+var reduceOpType = &operationType{Type: "REDUCE", NumArgs: 2, Precedence: 35, Handler: reduceOperator}
 
 var blockOpType = &operationType{Type: "BLOCK", Precedence: 10, NumArgs: 2, Handler: emptyOperator}
 

pkg/yqlib/operator_file_test.go

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

pkg/yqlib/operator_reduce.go

@@ -39,6 +39,8 @@ func reduceOperator(d *dataTreeNavigator, context Context, expressionNode *Expre
 		return Context{}, err
 	}
 
+	accum.SetVariable("context", context.MatchingNodes)
+
 	log.Debugf("with variable %v", variableName)
 
 	blockExp := expressionNode.Rhs.Rhs

pkg/yqlib/operator_reduce_test.go

@@ -21,6 +21,26 @@ var reduceOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!map)::Cathy: apples\nBob: bananas\n",
 		},
 	},
+	{
+		description:    "Merge all documents together - using context",
+		subdescription: "The _$context_ variable set by reduce lets you access the data outside the reduce block.",
+		document:       `a: cat`,
+		document2:      `b: dog`,
+		expression:     `fileIndex as $item ireduce ({}; . * ($context | select(fileIndex==$item)) )`,
+		expected: []string{
+			"D0, P[], (!!map)::a: cat\nb: dog\n",
+		},
+	},
+	{
+		description:    "Merge all documents together - without using context",
+		subdescription: "`$context` is just a convenient variable that `reduce` sets, you can use your own for more control",
+		document:       `c: {a: cat}`,
+		document2:      `c: {b: dog}`,
+		expression:     `.c as $root | fileIndex as $item ireduce ({}; . * ($root | select(fileIndex==$item)) )`,
+		expected: []string{
+			"D0, P[], (!!map)::{a: cat, b: dog}\n",
+		},
+	},
 }
 
 func TestReduceOperatorScenarios(t *testing.T) {