diff --git a/ast/ast.go b/ast/ast.go
index 415fad4..795f525 100644
--- a/ast/ast.go
+++ b/ast/ast.go
@@ -191,13 +191,18 @@ type Node interface {
 	addReadLen(int)
 	// clean read length
 	clearLen()
+}
+
+// MapKeyNode type for map key node
+type MapKeyNode interface {
+	Node
 	// String node to text without comment
 	stringWithoutComment() string
 }
 
 // ScalarNode type for scalar node
 type ScalarNode interface {
-	Node
+	MapKeyNode
 	GetValue() interface{}
 }
 
@@ -274,7 +279,7 @@ func readNode(p []byte, node Node) (int, error) {
 }
 
 // Null create node for null value
-func Null(tk *token.Token) Node {
+func Null(tk *token.Token) *NullNode {
 	return &NullNode{
 		BaseNode: &BaseNode{},
 		Token:    tk,
@@ -282,7 +287,7 @@ func Null(tk *token.Token) Node {
 }
 
 // Bool create node for boolean value
-func Bool(tk *token.Token) Node {
+func Bool(tk *token.Token) *BoolNode {
 	b, _ := strconv.ParseBool(tk.Value)
 	return &BoolNode{
 		BaseNode: &BaseNode{},
@@ -292,7 +297,7 @@ func Bool(tk *token.Token) Node {
 }
 
 // Integer create node for integer value
-func Integer(tk *token.Token) Node {
+func Integer(tk *token.Token) *IntegerNode {
 	value := removeUnderScoreFromNumber(tk.Value)
 	switch tk.Type {
 	case token.BinaryIntegerType:
@@ -386,7 +391,7 @@ func Integer(tk *token.Token) Node {
 }
 
 // Float create node for float value
-func Float(tk *token.Token) Node {
+func Float(tk *token.Token) *FloatNode {
 	f, _ := strconv.ParseFloat(removeUnderScoreFromNumber(tk.Value), 64)
 	return &FloatNode{
 		BaseNode: &BaseNode{},
@@ -467,7 +472,7 @@ func Mapping(tk *token.Token, isFlowStyle bool, values ...*MappingValueNode) *Ma
 }
 
 // MappingValue create node for mapping value
-func MappingValue(tk *token.Token, key Node, value Node) *MappingValueNode {
+func MappingValue(tk *token.Token, key MapKeyNode, value Node) *MappingValueNode {
 	return &MappingValueNode{
 		BaseNode: &BaseNode{},
 		Start:    tk,
@@ -564,10 +569,6 @@ func (f *File) String() string {
 	return strings.Join(docs, "\n")
 }
 
-func (f *File) stringWithoutComment() string {
-	return f.String()
-}
-
 // DocumentNode type of Document
 type DocumentNode struct {
 	*BaseNode
@@ -609,10 +610,6 @@ func (d *DocumentNode) String() string {
 	return strings.Join(doc, "\n")
 }
 
-func (d *DocumentNode) stringWithoutComment() string {
-	return d.String()
-}
-
 // MarshalYAML encodes to a YAML text
 func (d *DocumentNode) MarshalYAML() ([]byte, error) {
 	return []byte(d.String()), nil
@@ -956,7 +953,7 @@ func (n *MergeKeyNode) GetValue() interface{} {
 
 // String returns '<<' value
 func (n *MergeKeyNode) String() string {
-	return n.Token.Value
+	return n.stringWithoutComment()
 }
 
 func (n *MergeKeyNode) stringWithoutComment() string {
@@ -1137,7 +1134,7 @@ func (m *MapNodeIter) Next() bool {
 }
 
 // Key returns the key of the iterator's current map node entry.
-func (m *MapNodeIter) Key() Node {
+func (m *MapNodeIter) Key() MapKeyNode {
 	return m.values[m.idx].Key
 }
 
@@ -1260,14 +1257,6 @@ func (n *MappingNode) String() string {
 	return n.blockStyleString(commentMode)
 }
 
-func (n *MappingNode) stringWithoutComment() string {
-	commentMode := false
-	if n.IsFlowStyle || len(n.Values) == 0 {
-		return n.flowStyleString(commentMode)
-	}
-	return n.blockStyleString(commentMode)
-}
-
 // MapRange implements MapNode protocol
 func (n *MappingNode) MapRange() *MapNodeIter {
 	return &MapNodeIter{
@@ -1311,7 +1300,7 @@ func (n *MappingKeyNode) AddColumn(col int) {
 
 // String tag to text
 func (n *MappingKeyNode) String() string {
-	return fmt.Sprintf("%s %s", n.Start.Value, n.Value.String())
+	return n.stringWithoutComment()
 }
 
 func (n *MappingKeyNode) stringWithoutComment() string {
@@ -1327,7 +1316,7 @@ func (n *MappingKeyNode) MarshalYAML() ([]byte, error) {
 type MappingValueNode struct {
 	*BaseNode
 	Start *token.Token
-	Key   Node
+	Key   MapKeyNode
 	Value Node
 }
 
@@ -1434,26 +1423,6 @@ func (n *MappingValueNode) toString() string {
 	return fmt.Sprintf("%s%s:\n%s", space, n.Key.String(), n.Value.String())
 }
 
-func (n *MappingValueNode) stringWithoutComment() string {
-	space := strings.Repeat(" ", n.Key.GetToken().Position.Column-1)
-	keyIndentLevel := n.Key.GetToken().Position.IndentLevel
-	valueIndentLevel := n.Value.GetToken().Position.IndentLevel
-	if _, ok := n.Value.(ScalarNode); ok {
-		return fmt.Sprintf("%s%s: %s", space, n.Key.String(), n.Value.String())
-	} else if keyIndentLevel < valueIndentLevel {
-		return fmt.Sprintf("%s%s:\n%s", space, n.Key.String(), n.Value.String())
-	} else if m, ok := n.Value.(*MappingNode); ok && (m.IsFlowStyle || len(m.Values) == 0) {
-		return fmt.Sprintf("%s%s: %s", space, n.Key.String(), n.Value.String())
-	} else if s, ok := n.Value.(*SequenceNode); ok && (s.IsFlowStyle || len(s.Values) == 0) {
-		return fmt.Sprintf("%s%s: %s", space, n.Key.String(), n.Value.String())
-	} else if _, ok := n.Value.(*AnchorNode); ok {
-		return fmt.Sprintf("%s%s: %s", space, n.Key.String(), n.Value.String())
-	} else if _, ok := n.Value.(*AliasNode); ok {
-		return fmt.Sprintf("%s%s: %s", space, n.Key.String(), n.Value.String())
-	}
-	return fmt.Sprintf("%s%s:\n%s", space, n.Key.String(), n.Value.String())
-}
-
 // MapRange implements MapNode protocol
 func (n *MappingValueNode) MapRange() *MapNodeIter {
 	return &MapNodeIter{
@@ -1619,13 +1588,6 @@ func (n *SequenceNode) String() string {
 	return n.blockStyleString()
 }
 
-func (n *SequenceNode) stringWithoutComment() string {
-	if n.IsFlowStyle || len(n.Values) == 0 {
-		return n.flowStyleString()
-	}
-	return n.blockStyleString()
-}
-
 // ArrayRange implements ArrayNode protocol
 func (n *SequenceNode) ArrayRange() *ArrayNodeIter {
 	return &ArrayNodeIter{
@@ -1696,10 +1658,6 @@ func (n *AnchorNode) String() string {
 	return fmt.Sprintf("&%s %s", n.Name.String(), value)
 }
 
-func (n *AnchorNode) stringWithoutComment() string {
-	return n.String()
-}
-
 // MarshalYAML encodes to a YAML text
 func (n *AnchorNode) MarshalYAML() ([]byte, error) {
 	return []byte(n.String()), nil
@@ -1750,10 +1708,6 @@ func (n *AliasNode) String() string {
 	return fmt.Sprintf("*%s", n.Value.String())
 }
 
-func (n *AliasNode) stringWithoutComment() string {
-	return fmt.Sprintf("*%s", n.Value.String())
-}
-
 // MarshalYAML encodes to a YAML text
 func (n *AliasNode) MarshalYAML() ([]byte, error) {
 	return []byte(n.String()), nil
@@ -1791,10 +1745,6 @@ func (n *DirectiveNode) String() string {
 	return fmt.Sprintf("%s%s", n.Start.Value, n.Value.String())
 }
 
-func (n *DirectiveNode) stringWithoutComment() string {
-	return fmt.Sprintf("%s%s", n.Start.Value, n.Value.String())
-}
-
 // MarshalYAML encodes to a YAML text
 func (n *DirectiveNode) MarshalYAML() ([]byte, error) {
 	return []byte(n.String()), nil
@@ -1833,10 +1783,6 @@ func (n *TagNode) String() string {
 	return fmt.Sprintf("%s %s", n.Start.Value, n.Value.String())
 }
 
-func (n *TagNode) stringWithoutComment() string {
-	return fmt.Sprintf("%s %s", n.Start.Value, n.Value.String())
-}
-
 // MarshalYAML encodes to a YAML text
 func (n *TagNode) MarshalYAML() ([]byte, error) {
 	return []byte(n.String()), nil
@@ -1872,10 +1818,6 @@ func (n *CommentNode) String() string {
 	return fmt.Sprintf("#%s", n.Token.Value)
 }
 
-func (n *CommentNode) stringWithoutComment() string {
-	return ""
-}
-
 // MarshalYAML encodes to a YAML text
 func (n *CommentNode) MarshalYAML() ([]byte, error) {
 	return []byte(n.String()), nil
@@ -1929,10 +1871,6 @@ func (n *CommentGroupNode) StringWithSpace(col int) string {
 
 }
 
-func (n *CommentGroupNode) stringWithoutComment() string {
-	return ""
-}
-
 // MarshalYAML encodes to a YAML text
 func (n *CommentGroupNode) MarshalYAML() ([]byte, error) {
 	return []byte(n.String()), nil
diff --git a/decode.go b/decode.go
index d519b78..2682b2a 100644
--- a/decode.go
+++ b/decode.go
@@ -104,7 +104,7 @@ func (d *Decoder) mergeValueNode(value ast.Node) ast.Node {
 	return value
 }
 
-func (d *Decoder) mapKeyNodeToString(node ast.Node) string {
+func (d *Decoder) mapKeyNodeToString(node ast.MapKeyNode) string {
 	key := d.nodeToValue(node)
 	if key == nil {
 		return "null"
@@ -283,7 +283,7 @@ func (d *Decoder) resolveAlias(node ast.Node) ast.Node {
 			value.AddColumn(requiredColumn)
 			n.Value = value
 		} else {
-			n.Key = d.resolveAlias(n.Key)
+			n.Key = d.resolveAlias(n.Key).(ast.MapKeyNode)
 			n.Value = d.resolveAlias(n.Value)
 		}
 	case *ast.SequenceNode:
diff --git a/encode.go b/encode.go
index 31eaf94..f582b48 100644
--- a/encode.go
+++ b/encode.go
@@ -351,7 +351,6 @@ func (e *Encoder) encodeValue(ctx context.Context, v reflect.Value, column int)
 	default:
 		return nil, xerrors.Errorf("unknown value type %s", v.Type().String())
 	}
-	return nil, nil
 }
 
 func (e *Encoder) pos(column int) *token.Position {
@@ -364,17 +363,17 @@ func (e *Encoder) pos(column int) *token.Position {
 	}
 }
 
-func (e *Encoder) encodeNil() ast.Node {
+func (e *Encoder) encodeNil() *ast.NullNode {
 	value := "null"
 	return ast.Null(token.New(value, value, e.pos(e.column)))
 }
 
-func (e *Encoder) encodeInt(v int64) ast.Node {
+func (e *Encoder) encodeInt(v int64) *ast.IntegerNode {
 	value := fmt.Sprint(v)
 	return ast.Integer(token.New(value, value, e.pos(e.column)))
 }
 
-func (e *Encoder) encodeUint(v uint64) ast.Node {
+func (e *Encoder) encodeUint(v uint64) *ast.IntegerNode {
 	value := fmt.Sprint(v)
 	return ast.Integer(token.New(value, value, e.pos(e.column)))
 }
@@ -411,7 +410,7 @@ func (e *Encoder) isNeedQuoted(v string) bool {
 	return false
 }
 
-func (e *Encoder) encodeString(v string, column int) ast.Node {
+func (e *Encoder) encodeString(v string, column int) *ast.StringNode {
 	if e.isNeedQuoted(v) {
 		if e.singleQuote {
 			v = quoteWith(v, '\'')
@@ -422,12 +421,12 @@ func (e *Encoder) encodeString(v string, column int) ast.Node {
 	return ast.String(token.New(v, v, e.pos(column)))
 }
 
-func (e *Encoder) encodeBool(v bool) ast.Node {
+func (e *Encoder) encodeBool(v bool) *ast.BoolNode {
 	value := fmt.Sprint(v)
 	return ast.Bool(token.New(value, value, e.pos(e.column)))
 }
 
-func (e *Encoder) encodeSlice(ctx context.Context, value reflect.Value) (ast.Node, error) {
+func (e *Encoder) encodeSlice(ctx context.Context, value reflect.Value) (*ast.SequenceNode, error) {
 	if e.indentSequence {
 		e.column += e.indent
 	}
@@ -446,7 +445,7 @@ func (e *Encoder) encodeSlice(ctx context.Context, value reflect.Value) (ast.Nod
 	return sequence, nil
 }
 
-func (e *Encoder) encodeArray(ctx context.Context, value reflect.Value) (ast.Node, error) {
+func (e *Encoder) encodeArray(ctx context.Context, value reflect.Value) (*ast.SequenceNode, error) {
 	if e.indentSequence {
 		e.column += e.indent
 	}
@@ -482,7 +481,7 @@ func (e *Encoder) encodeMapItem(ctx context.Context, item MapItem, column int) (
 	), nil
 }
 
-func (e *Encoder) encodeMapSlice(ctx context.Context, value MapSlice, column int) (ast.Node, error) {
+func (e *Encoder) encodeMapSlice(ctx context.Context, value MapSlice, column int) (*ast.MappingNode, error) {
 	node := ast.Mapping(token.New("", "", e.pos(column)), e.isFlowStyle)
 	for _, item := range value {
 		value, err := e.encodeMapItem(ctx, item, column)
@@ -569,7 +568,7 @@ func (e *Encoder) isZeroValue(v reflect.Value) bool {
 	return false
 }
 
-func (e *Encoder) encodeTime(v time.Time, column int) ast.Node {
+func (e *Encoder) encodeTime(v time.Time, column int) *ast.StringNode {
 	value := v.Format(time.RFC3339Nano)
 	if e.isJSONStyle {
 		value = strconv.Quote(value)
@@ -577,7 +576,7 @@ func (e *Encoder) encodeTime(v time.Time, column int) ast.Node {
 	return ast.String(token.New(value, value, e.pos(column)))
 }
 
-func (e *Encoder) encodeDuration(v time.Duration, column int) ast.Node {
+func (e *Encoder) encodeDuration(v time.Duration, column int) *ast.StringNode {
 	value := v.String()
 	if e.isJSONStyle {
 		value = strconv.Quote(value)
@@ -585,7 +584,7 @@ func (e *Encoder) encodeDuration(v time.Duration, column int) ast.Node {
 	return ast.String(token.New(value, value, e.pos(column)))
 }
 
-func (e *Encoder) encodeAnchor(anchorName string, value ast.Node, fieldValue reflect.Value, column int) (ast.Node, error) {
+func (e *Encoder) encodeAnchor(anchorName string, value ast.Node, fieldValue reflect.Value, column int) (*ast.AnchorNode, error) {
 	anchorNode := ast.Anchor(token.New("&", "&", e.pos(column)))
 	anchorNode.Name = ast.String(token.New(anchorName, anchorName, e.pos(column)))
 	anchorNode.Value = value
@@ -637,7 +636,7 @@ func (e *Encoder) encodeStruct(ctx context.Context, value reflect.Value, column
 				s.SetIsFlowStyle(true)
 			}
 		}
-		key := e.encodeString(structField.RenderName, column)
+		var key ast.MapKeyNode = e.encodeString(structField.RenderName, column)
 		switch {
 		case structField.AnchorName != "":
 			anchorNode, err := e.encodeAnchor(structField.AnchorName, value, fieldValue, column)
diff --git a/parser/parser.go b/parser/parser.go
index 80c9f51..70937c9 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -14,7 +14,7 @@ import (
 
 type parser struct{}
 
-func (p *parser) parseMapping(ctx *context) (ast.Node, error) {
+func (p *parser) parseMapping(ctx *context) (*ast.MappingNode, error) {
 	mapTk := ctx.currentToken()
 	node := ast.Mapping(mapTk, true)
 	node.SetPath(ctx.path)
@@ -43,7 +43,7 @@ func (p *parser) parseMapping(ctx *context) (ast.Node, error) {
 	return nil, errors.ErrSyntax("unterminated flow mapping", node.GetToken())
 }
 
-func (p *parser) parseSequence(ctx *context) (ast.Node, error) {
+func (p *parser) parseSequence(ctx *context) (*ast.SequenceNode, error) {
 	node := ast.Sequence(ctx.currentToken(), true)
 	node.SetPath(ctx.path)
 	ctx.progress(1) // skip SequenceStart token
@@ -67,7 +67,7 @@ func (p *parser) parseSequence(ctx *context) (ast.Node, error) {
 	return node, nil
 }
 
-func (p *parser) parseTag(ctx *context) (ast.Node, error) {
+func (p *parser) parseTag(ctx *context) (*ast.TagNode, error) {
 	tagToken := ctx.currentToken()
 	node := ast.Tag(tagToken)
 	node.SetPath(ctx.path)
@@ -138,7 +138,7 @@ func (p *parser) createNullToken(base *token.Token) *token.Token {
 	return token.New("null", "null", &pos)
 }
 
-func (p *parser) parseMapValue(ctx *context, key ast.Node, colonToken *token.Token) (ast.Node, error) {
+func (p *parser) parseMapValue(ctx *context, key ast.MapKeyNode, colonToken *token.Token) (ast.Node, error) {
 	node, err := p.createMapValueNode(ctx, key, colonToken)
 	if err != nil {
 		return nil, errors.Wrapf(err, "failed to create map value node")
@@ -149,7 +149,7 @@ func (p *parser) parseMapValue(ctx *context, key ast.Node, colonToken *token.Tok
 	return node, nil
 }
 
-func (p *parser) createMapValueNode(ctx *context, key ast.Node, colonToken *token.Token) (ast.Node, error) {
+func (p *parser) createMapValueNode(ctx *context, key ast.MapKeyNode, colonToken *token.Token) (ast.Node, error) {
 	tk := ctx.currentToken()
 	if tk == nil {
 		nullToken := p.createNullToken(colonToken)
@@ -273,7 +273,7 @@ func (p *parser) parseMappingValue(ctx *context) (ast.Node, error) {
 	return node, nil
 }
 
-func (p *parser) parseSequenceEntry(ctx *context) (ast.Node, error) {
+func (p *parser) parseSequenceEntry(ctx *context) (*ast.SequenceNode, error) {
 	tk := ctx.currentToken()
 	sequenceNode := ast.Sequence(tk, false)
 	sequenceNode.SetPath(ctx.path)
@@ -319,7 +319,7 @@ func (p *parser) parseSequenceEntry(ctx *context) (ast.Node, error) {
 	return sequenceNode, nil
 }
 
-func (p *parser) parseAnchor(ctx *context) (ast.Node, error) {
+func (p *parser) parseAnchor(ctx *context) (*ast.AnchorNode, error) {
 	tk := ctx.currentToken()
 	anchor := ast.Anchor(tk)
 	anchor.SetPath(ctx.path)
@@ -346,7 +346,7 @@ func (p *parser) parseAnchor(ctx *context) (ast.Node, error) {
 	return anchor, nil
 }
 
-func (p *parser) parseAlias(ctx *context) (ast.Node, error) {
+func (p *parser) parseAlias(ctx *context) (*ast.AliasNode, error) {
 	tk := ctx.currentToken()
 	alias := ast.Alias(tk)
 	alias.SetPath(ctx.path)
@@ -363,7 +363,7 @@ func (p *parser) parseAlias(ctx *context) (ast.Node, error) {
 	return alias, nil
 }
 
-func (p *parser) parseMapKey(ctx *context) (ast.Node, error) {
+func (p *parser) parseMapKey(ctx *context) (ast.MapKeyNode, error) {
 	tk := ctx.currentToken()
 	if value := p.parseScalarValue(tk); value != nil {
 		return value, nil
@@ -377,7 +377,7 @@ func (p *parser) parseMapKey(ctx *context) (ast.Node, error) {
 	return nil, errors.ErrSyntax("unexpected mapping key", tk)
 }
 
-func (p *parser) parseStringValue(tk *token.Token) ast.Node {
+func (p *parser) parseStringValue(tk *token.Token) *ast.StringNode {
 	switch tk.Type {
 	case token.StringType,
 		token.SingleQuoteType,
@@ -387,7 +387,7 @@ func (p *parser) parseStringValue(tk *token.Token) ast.Node {
 	return nil
 }
 
-func (p *parser) parseScalarValueWithComment(ctx *context, tk *token.Token) (ast.Node, error) {
+func (p *parser) parseScalarValueWithComment(ctx *context, tk *token.Token) (ast.ScalarNode, error) {
 	node := p.parseScalarValue(tk)
 	if node == nil {
 		return nil, nil
@@ -402,7 +402,7 @@ func (p *parser) parseScalarValueWithComment(ctx *context, tk *token.Token) (ast
 	return node, nil
 }
 
-func (p *parser) parseScalarValue(tk *token.Token) ast.Node {
+func (p *parser) parseScalarValue(tk *token.Token) ast.ScalarNode {
 	if node := p.parseStringValue(tk); node != nil {
 		return node
 	}
@@ -426,7 +426,7 @@ func (p *parser) parseScalarValue(tk *token.Token) ast.Node {
 	return nil
 }
 
-func (p *parser) parseDirective(ctx *context) (ast.Node, error) {
+func (p *parser) parseDirective(ctx *context) (*ast.DirectiveNode, error) {
 	node := ast.Directive(ctx.currentToken())
 	ctx.progress(1) // skip directive token
 	value, err := p.parseToken(ctx, ctx.currentToken())
@@ -447,7 +447,7 @@ func (p *parser) parseDirective(ctx *context) (ast.Node, error) {
 	return node, nil
 }
 
-func (p *parser) parseLiteral(ctx *context) (ast.Node, error) {
+func (p *parser) parseLiteral(ctx *context) (*ast.LiteralNode, error) {
 	node := ast.Literal(ctx.currentToken())
 	ctx.progress(1) // skip literal/folded token
 
@@ -543,7 +543,7 @@ func (p *parser) parseComment(ctx *context) (ast.Node, error) {
 	return node, nil
 }
 
-func (p *parser) parseMappingKey(ctx *context) (ast.Node, error) {
+func (p *parser) parseMappingKey(ctx *context) (*ast.MappingKeyNode, error) {
 	keyTk := ctx.currentToken()
 	node := ast.MappingKey(keyTk)
 	node.SetPath(ctx.path)