Added Label combinator

gigaparsec.go

@@ -21,6 +21,11 @@ type Message struct {
 	Expected []string
 }
 
+func (m Message) expect(s string) Message {
+	m.Expected = []string{s}
+	return m
+}
+
 func MessageOK(pos uint64) Message { return Message{Pos: pos} }
 
 func MessageEnd(pos uint64) Message { return Message{Pos: pos, Got: "end of input"} }
@@ -189,6 +194,25 @@ func Try[In, Out any](p Parser[In, Out]) Parser[In, Out] {
 	}
 }
 
+func Label[In, Out any](p Parser[In, Out], l string) Parser[In, Out] {
+	return func(input State[In]) (consumed bool, reply Result[In, Out], err error) {
+		consumed, reply, err = p(input)
+		if consumed {
+			return
+		}
+		if err == nil {
+			reply.Message = reply.Message.expect(l)
+			return
+		}
+		var parseErr ParseError
+		if errors.As(err, &parseErr) {
+			err = ParseError(Message(parseErr).expect(l))
+			return
+		}
+		return
+	}
+}
+
 func Map[In, Out1, Out2 any](p Parser[In, Out1], f func(Out1) Out2) Parser[In, Out2] {
 	return Bind(p, func(out Out1) Parser[In, Out2] {
 		return Return[In](f(out))