replace collection start with a function; fix regression

the last cl had an unnecessary second interface; rewind slightly to use a function for the collection iteration generation.
also: fix a crash bug when comments are missing in a map which wants to encode comments ( regression from 0.7 behavior )

encode/encode.go

@@ -14,30 +14,28 @@ import (
 // an encoder that expects no comments
 func MakeEncoder(w io.Writer) Encoder {
 	var m MapTransform
-	var n SequenceTransform
 	return Encoder{
 		Tabs:      TabWriter{Writer: w},
 		Mapper:    m.Mapper(),
-		Sequencer: n.Sequencer(),
+		Sequencer: MakeSequence,
 	}
 }
 
 // use the "CommentBlock" encoder
 func MakeCommentEncoder(w io.Writer) Encoder {
 	var m MapTransform
-	var n SequenceTransform
 	return Encoder{
 		Tabs:             TabWriter{Writer: w},
 		Mapper:           m.Mapper(),
-		Sequencer:        n.Sequencer(),
+		Sequencer:        MakeSequence,
 		MapComments:      CommentBlock,
 		SequenceComments: CommentBlock,
 	}
 }
 
 type Encoder struct {
 	Tabs              TabWriter
-	Mapper, Sequencer Collection
+	Mapper, Sequencer StartCollection
 	MapComments       Commenting
 	SequenceComments  Commenting
 }
@@ -125,7 +123,7 @@ func (enc *Encoder) WriteValue(v r.Value, wasMaps bool) (err error) {
 
 			case r.Array, r.Slice:
 				// tbd: look at tag for "want array"?
-				if it, e := enc.Sequencer.StartCollection(v); e != nil {
+				if it, e := enc.Sequencer(v); e != nil {
 					err = e
 				} else if it == nil {
 					tab.WriteRune(runes.ArrayOpen)
@@ -135,7 +133,7 @@ func (enc *Encoder) WriteValue(v r.Value, wasMaps bool) (err error) {
 				}
 
 			case r.Map:
-				if it, e := enc.Mapper.StartCollection(v); e != nil {
+				if it, e := enc.Mapper(v); e != nil {
 					err = e
 				} else if it != nil {
 					err = enc.WriteMapping(it, wasMaps)
@@ -180,10 +178,8 @@ func (enc *Encoder) writeCollection(it Iterator, cmts Commenting, wasMaps, maps
 	}
 
 	// setup a comment iterator:
-	var cit Comments
-	if cmts == nil {
-		cit = noComments{} // expect none by default
-	} else {
+	var cit Comments = noComments{} // expect none by default
+	if cmts != nil {
 		key, val := it.GetKey(), getValue(it)
 		if !maps || len(key) == 0 {
 			cit, err = cmts(val)

encode/encodeCollect.go

@@ -2,11 +2,9 @@ package encode
 
 import r "reflect"
 
-type Collection interface {
-	// for sequences, value is guaranteed to be a reflect.Slice
-	// for mappings, value is guaranteed to be a reflect.Map
-	StartCollection(r.Value) (Iterator, error)
-}
+// for sequences, value is guaranteed to be a reflect.Slice
+// for mappings, value is guaranteed to be a reflect.Map
+type StartCollection func(r.Value) (Iterator, error)
 
 // controls serialization when implemented by a value that's being encoded
 type TellMapping interface {
@@ -48,10 +46,3 @@ type Comments interface {
 	Next() bool          // called before every element, false if there are no more elements
 	GetComment() Comment // valid after next returns true
 }
-
-// implements StartCollection using a function
-type sequenceStarter func(r.Value) (Iterator, error)
-
-func (q sequenceStarter) StartCollection(v r.Value) (Iterator, error) {
-	return q(v)
-}

encode/encodeComments.go

@@ -17,6 +17,9 @@ func CommentSlice(els []Comment) Comments {
 	return &commentSlice{next: els}
 }
 
+// a nil commenting implementation for readability
+var NoComments Commenting = nil
+
 // implies that there are comments;
 // the encoder should simply skip them.
 func DiscardComments(r.Value) (Comments, error) {

encode/encodeMap.go

@@ -14,8 +14,8 @@ type MapTransform struct {
 }
 
 // return a factory function for the encoder
-func (m *MapTransform) Mapper() Collection {
-	return sequenceStarter(m.makeMapping)
+func (m *MapTransform) Mapper() StartCollection {
+	return m.makeMapping
 }
 
 // sort keys; by default keys are written sorted as per standard go string rules.

encode/encodeSeq.go

@@ -5,19 +5,12 @@ import (
 	r "reflect"
 )
 
-type SequenceTransform struct{}
-
-// return a factory function for the encoder
-func (n *SequenceTransform) Sequencer() Collection {
-	return sequenceStarter(n.makeSequence)
-}
-
 // todo? sort values; by default sequences are not sorted
 // func (m *SequenceTransform) Sort(t func(a, b r.Value) bool) {
 // 	m.sort = t
 // }
 
-func (n *SequenceTransform) makeSequence(src r.Value) (ret Iterator, err error) {
+func MakeSequence(src r.Value) (ret Iterator, err error) {
 	if e := validateSeq(src); e != nil {
 		err = e
 	} else if cnt := src.Len(); cnt > 0 {

encoder.go

@@ -26,7 +26,7 @@ func (enc *Encoder) Encode(v any) (err error) {
 
 // configure how mappings are encoded
 // returns self for chaining
-func (enc *Encoder) SetMapper(n encode.Collection, c encode.Commenting) *Encoder {
+func (enc *Encoder) SetMapper(n encode.StartCollection, c encode.Commenting) *Encoder {
 	inner := (*encode.Encoder)(enc)
 	inner.Mapper = n
 	inner.MapComments = c
@@ -35,7 +35,7 @@ func (enc *Encoder) SetMapper(n encode.Collection, c encode.Commenting) *Encoder
 
 // configure how sequences are encoded
 // returns self for chaining
-func (enc *Encoder) SetSequencer(n encode.Collection, c encode.Commenting) *Encoder {
+func (enc *Encoder) SetSequencer(n encode.StartCollection, c encode.Commenting) *Encoder {
 	inner := (*encode.Encoder)(enc)
 	inner.Sequencer = n
 	inner.SequenceComments = c