Fixed bug in alternative op, dont evaluate RHS if LHS is truthy

Author: mikefarah

pkg/yqlib/doc/operators/alternative-default-value.md

@@ -79,3 +79,36 @@ will output
 cat
 ```
 
+## Update or create - entity exists
+This initialises `a` if it's not present
+
+Given a sample.yml file of:
+```yaml
+a: 1
+```
+then
+```bash
+yq '(.a // (.a = 0)) += 1' sample.yml
+```
+will output
+```yaml
+a: 2
+```
+
+## Update or create - entity does not exist
+This initialises `a` if it's not present
+
+Given a sample.yml file of:
+```yaml
+b: camel
+```
+then
+```bash
+yq '(.a // (.a = 0)) += 1' sample.yml
+```
+will output
+```yaml
+b: camel
+a: 1
+```
+

pkg/yqlib/operator_alternative.go

@@ -2,7 +2,24 @@ package yqlib
 
 func alternativeOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
 	log.Debugf("-- alternative")
-	return crossFunction(d, context, expressionNode, alternativeFunc, true)
+	prefs := crossFunctionPreferences{
+		CalcWhenEmpty: true,
+		Calculation:   alternativeFunc,
+		LhsResultValue: func(lhs *CandidateNode) (*CandidateNode, error) {
+			if lhs == nil {
+				return nil, nil
+			}
+			truthy, err := isTruthy(lhs)
+			if err != nil {
+				return nil, err
+			}
+			if truthy {
+				return lhs, nil
+			}
+			return nil, nil
+		},
+	}
+	return crossFunctionWithPrefs(d, context, expressionNode, prefs)
 }
 
 func alternativeFunc(d *dataTreeNavigator, context Context, lhs *CandidateNode, rhs *CandidateNode) (*CandidateNode, error) {

pkg/yqlib/operator_alternative_test.go

@@ -85,6 +85,24 @@ var alternativeOperatorScenarios = []expressionScenario{
 			"D0, P[], (!!bool)::true\n",
 		},
 	},
+	{
+		description:    "Update or create - entity exists",
+		subdescription: "This initialises `a` if it's not present",
+		expression:     "(.a // (.a = 0)) += 1",
+		document:       `a: 1`,
+		expected: []string{
+			"D0, P[], (doc)::a: 2\n",
+		},
+	},
+	{
+		description:    "Update or create - entity does not exist",
+		subdescription: "This initialises `a` if it's not present",
+		expression:     "(.a // (.a = 0)) += 1",
+		document:       `b: camel`,
+		expected: []string{
+			"D0, P[], (!!map)::b: camel\na: 1\n",
+		},
+	},
 }
 
 func TestAlternativeOperatorScenarios(t *testing.T) {