encoder: add option to drop merge tag (#14)

The !!merge tag is superfluous for most people. Need an option to drop
it when it's unnecessary.

encode.go

@@ -29,12 +29,13 @@ import (
 )
 
 type encoder struct {
-	emitter  yaml_emitter_t
-	event    yaml_event_t
-	out      []byte
-	flow     bool
-	indent   int
-	doneInit bool
+	emitter         yaml_emitter_t
+	event           yaml_event_t
+	out             []byte
+	flow            bool
+	indent          int
+	doneInit        bool
+	optDropMergeTag bool
 }
 
 func newEncoder() *encoder {
@@ -468,6 +469,10 @@ func (e *encoder) node(node *Node, tail string) {
 		}
 	}
 
+	if tag == mergeTag && e.optDropMergeTag {
+		tag = ""
+	}
+
 	switch node.Kind {
 	case DocumentNode:
 		yaml_document_start_event_initialize(&e.event, nil, nil, true)

formattest/encode_test.go

@@ -51,3 +51,14 @@ func TestBlockScalar(t *testing.T) {
 		},
 	}.Run(t)
 }
+
+func TestDropMergeTag(t *testing.T) {
+	formatTestCase{
+		name:             "drop merge tag",
+		folder:           "drop_merge_tag",
+		configureDecoder: noopDecoder,
+		configureEncoder: func(enc *yaml.Encoder) {
+			enc.SetDropMergeTag(true)
+		},
+	}.Run(t)
+}

formattest/testdata/drop_merge_tag/expected.yaml

@@ -0,0 +1,3 @@
+a: &a
+b:
+    <<: *a

formattest/testdata/drop_merge_tag/input.yaml

@@ -0,0 +1,3 @@
+a: &a
+b:
+    <<: *a

yaml.go

@@ -318,6 +318,11 @@ func (e *Encoder) SetIndentlessBlockSequence(indentlessBlockSequence bool) {
 	yaml_emitter_set_indentless_block_sequence(&e.encoder.emitter, indentlessBlockSequence)
 }
 
+// SetDropMergeTag sets optDropMergeTag on the encoder.
+func (e *Encoder) SetDropMergeTag(dropMergeTag bool) {
+	e.encoder.optDropMergeTag = dropMergeTag
+}
+
 // Close closes the encoder by writing any remaining data.
 // It does not write a stream terminating string "...".
 func (e *Encoder) Close() (err error) {