Add Bind for constants

combinator.go

@@ -123,6 +123,19 @@ func Maybe(parser Parserish) Parser {
 	})
 }
 
+func Bind(parser Parserish, val interface{}) Parser {
+	p := Parsify(parser)
+
+	return func(ps *State) Node {
+		node := p(ps)
+		if ps.Errored() {
+			return node
+		}
+		node.Result = val
+		return node
+	}
+}
+
 func Map(parser Parserish, f func(n Node) Node) Parser {
 	p := Parsify(parser)
 

json/json.go

@@ -27,25 +27,13 @@ var (
 		return Node{Result: ret}
 	})
 
-	_null = Map("null", func(n Node) Node {
-		return Node{Result: nil}
-	})
-
-	_true = Map("true", func(n Node) Node {
-		return Node{Result: true}
-	})
-
-	_false = Map("false", func(n Node) Node {
-		return Node{Result: false}
-	})
+	_null  = Bind("null", nil)
+	_true  = Bind("true", true)
+	_false = Bind("false", false)
 
 	_string = Map(String('"'), func(n Node) Node {
 		return Node{Result: n.Token}
 	})
-
-	Y = Map(&value, func(n Node) Node {
-		return Node{Result: n.Result}
-	})
 )
 
 func init() {
@@ -53,7 +41,7 @@ func init() {
 }
 
 func Unmarshal(input string) (interface{}, error) {
-	result, remaining, err := ParseString(Y, input)
+	result, remaining, err := ParseString(value, input)
 
 	if err != nil {
 		return result, err