Use a lot less MakeState

gigaparsec.go

@@ -133,7 +133,8 @@ func MessageEnd(pos uint64, expected ...string) Message {
 	return Message{pos: pos, got: "end of input", expected: expected}
 }
 
-// MakeState creates a new State
+// MakeState creates a new State.
+// It's mostly useful for testing. You probably don't need it.
 func MakeState[In any](r ReaderAt[In]) State[In] {
 	return State[In]{r: r}
 }
@@ -225,10 +226,15 @@ func (p Parser[In, Out]) Map[Out2 any](f func(Out) Out2) Parser[In, Out2] {
 	})
 }
 
-// Run executes a parser on r and returns the result.
-func (p Parser[In, Out]) Run(r ReaderAt[In]) (out Out, err error) {
+// RunToResult executes a parser on r and returns the result, or an error if reading the inputs fails.
+func (p Parser[In, Out]) RunToResult(r ReaderAt[In]) (Result[In, Out], error) {
 	start := MakeState(r)
-	result, err := p(start)
+	return p(start)
+}
+
+// Run executes a parser on r and returns the output, or an an error if the parse fails.
+func (p Parser[In, Out]) Run(r ReaderAt[In]) (out Out, err error) {
+	result, err := p.RunToResult(r)
 	if err != nil {
 		err = fmt.Errorf("Run: %w", err)
 		return