Adding first operator

Author: mikefarah

pkg/yqlib/doc/operators/first.md

@@ -0,0 +1,345 @@
+
+## First matching element from array
+Given a sample.yml file of:
+```yaml
+- a: banana
+- a: cat
+- a: apple
+```
+then
+```bash
+yq 'first(.a == "cat")' sample.yml
+```
+will output
+```yaml
+a: cat
+```
+
+## First matching element from array with multiple matches
+Given a sample.yml file of:
+```yaml
+- a: banana
+- a: cat
+- a: apple
+- a: cat
+```
+then
+```bash
+yq 'first(.a == "cat")' sample.yml
+```
+will output
+```yaml
+a: cat
+```
+
+## First matching element from array with numeric condition
+Given a sample.yml file of:
+```yaml
+- a: 10
+- a: 100
+- a: 1
+```
+then
+```bash
+yq 'first(.a > 50)' sample.yml
+```
+will output
+```yaml
+a: 100
+```
+
+## First matching element from array with boolean condition
+Given a sample.yml file of:
+```yaml
+- a: false
+- a: true
+- a: false
+```
+then
+```bash
+yq 'first(.a == true)' sample.yml
+```
+will output
+```yaml
+a: true
+```
+
+## First matching element from array with null values
+Given a sample.yml file of:
+```yaml
+- a: null
+- a: cat
+- a: apple
+```
+then
+```bash
+yq 'first(.a != null)' sample.yml
+```
+will output
+```yaml
+a: cat
+```
+
+## First matching element from array with complex condition
+Given a sample.yml file of:
+```yaml
+- a: dog
+  b: 5
+- a: cat
+  b: 3
+- a: apple
+  b: 7
+```
+then
+```bash
+yq 'first(.b > 4)' sample.yml
+```
+will output
+```yaml
+a: dog
+b: 5
+```
+
+## First matching element from map
+Given a sample.yml file of:
+```yaml
+x:
+  a: banana
+y:
+  a: cat
+z:
+  a: apple
+```
+then
+```bash
+yq 'first(.a == "cat")' sample.yml
+```
+will output
+```yaml
+a: cat
+```
+
+## First matching element from map with numeric condition
+Given a sample.yml file of:
+```yaml
+x:
+  a: 10
+y:
+  a: 100
+z:
+  a: 1
+```
+then
+```bash
+yq 'first(.a > 50)' sample.yml
+```
+will output
+```yaml
+a: 100
+```
+
+## First matching element from nested structure
+Given a sample.yml file of:
+```yaml
+items:
+  - a: banana
+  - a: cat
+  - a: apple
+```
+then
+```bash
+yq '.items | first(.a == "cat")' sample.yml
+```
+will output
+```yaml
+a: cat
+```
+
+## First matching element with no matches
+Given a sample.yml file of:
+```yaml
+- a: banana
+- a: cat
+- a: apple
+```
+then
+```bash
+yq 'first(.a == "dog")' sample.yml
+```
+will output
+```yaml
+```
+
+## First matching element from empty array
+Given a sample.yml file of:
+```yaml
+[]
+```
+then
+```bash
+yq 'first(.a == "cat")' sample.yml
+```
+will output
+```yaml
+```
+
+## First matching element from scalar node
+Given a sample.yml file of:
+```yaml
+hello
+```
+then
+```bash
+yq 'first(. == "hello")' sample.yml
+```
+will output
+```yaml
+```
+
+## First matching element from null node
+Given a sample.yml file of:
+```yaml
+null
+```
+then
+```bash
+yq 'first(. == "hello")' sample.yml
+```
+will output
+```yaml
+```
+
+## First matching element with string condition
+Given a sample.yml file of:
+```yaml
+- a: banana
+- a: cat
+- a: apple
+```
+then
+```bash
+yq 'first(.a | test("^c"))' sample.yml
+```
+will output
+```yaml
+a: cat
+```
+
+## First matching element with length condition
+Given a sample.yml file of:
+```yaml
+- a: hi
+- a: hello
+- a: world
+```
+then
+```bash
+yq 'first(.a | length > 4)' sample.yml
+```
+will output
+```yaml
+a: hello
+```
+
+## First matching element from array of strings
+Given a sample.yml file of:
+```yaml
+- banana
+- cat
+- apple
+```
+then
+```bash
+yq 'first(. == "cat")' sample.yml
+```
+will output
+```yaml
+cat
+```
+
+## First matching element from array of numbers
+Given a sample.yml file of:
+```yaml
+- 10
+- 100
+- 1
+```
+then
+```bash
+yq 'first(. > 50)' sample.yml
+```
+will output
+```yaml
+100
+```
+
+## First element with no RHS from array
+Given a sample.yml file of:
+```yaml
+- 10
+- 100
+- 1
+```
+then
+```bash
+yq 'first' sample.yml
+```
+will output
+```yaml
+10
+```
+
+## First element with no RHS from array of maps
+Given a sample.yml file of:
+```yaml
+- a: 10
+- a: 100
+```
+then
+```bash
+yq 'first' sample.yml
+```
+will output
+```yaml
+a: 10
+```
+
+## No RHS on empty array returns nothing
+Given a sample.yml file of:
+```yaml
+[]
+```
+then
+```bash
+yq 'first' sample.yml
+```
+will output
+```yaml
+```
+
+## No RHS on scalar returns nothing
+Given a sample.yml file of:
+```yaml
+hello
+```
+then
+```bash
+yq 'first' sample.yml
+```
+will output
+```yaml
+```
+
+## No RHS on null returns nothing
+Given a sample.yml file of:
+```yaml
+null
+```
+then
+```bash
+yq 'first' sample.yml
+```
+will output
+```yaml
+```
+

pkg/yqlib/expression_parser.go

@@ -54,6 +54,12 @@ func (p *expressionParserImpl) createExpressionTree(postFixPath []*Operation) (*
 			switch numArgs {
 			case 1:
 				if len(stack) < 1 {
+					// Allow certain unary ops to accept zero args by interpreting missing RHS as nil
+					// TODO - make this more general on OperationType
+					if Operation.OperationType == firstOpType {
+						// no RHS provided; proceed without popping
+						break
+					}
 					return nil, fmt.Errorf("'%v' expects 1 arg but received none", strings.TrimSpace(Operation.StringValue))
 				}
 				remaining, rhs := stack[:len(stack)-1], stack[len(stack)-1]

pkg/yqlib/lexer_participle.go

@@ -118,6 +118,7 @@ var participleYqRules = []*participleYqRule{
 
 	simpleOp("sort_?by", sortByOpType),
 	simpleOp("sort", sortOpType),
+	simpleOp("first", firstOpType),
 
 	simpleOp("reverse", reverseOpType),
 

pkg/yqlib/operation.go

@@ -143,6 +143,7 @@ var delPathsOpType = &operationType{Type: "DEL_PATHS", NumArgs: 1, Precedence: 5
 
 var explodeOpType = &operationType{Type: "EXPLODE", NumArgs: 1, Precedence: 52, Handler: explodeOperator, CheckForPostTraverse: true}
 var sortByOpType = &operationType{Type: "SORT_BY", NumArgs: 1, Precedence: 52, Handler: sortByOperator, CheckForPostTraverse: true}
+var firstOpType = &operationType{Type: "FIRST", NumArgs: 1, Precedence: 52, Handler: firstOperator, CheckForPostTraverse: true}
 var reverseOpType = &operationType{Type: "REVERSE", NumArgs: 0, Precedence: 52, Handler: reverseOperator, CheckForPostTraverse: true}
 var sortOpType = &operationType{Type: "SORT", NumArgs: 0, Precedence: 52, Handler: sortOperator, CheckForPostTraverse: true}
 var shuffleOpType = &operationType{Type: "SHUFFLE", NumArgs: 0, Precedence: 52, Handler: shuffleOperator, CheckForPostTraverse: true}