Added Bracket, Where, and Token (without tests)

TODO.txt

@@ -5,3 +5,5 @@ Document Seq
 Should MakeState be private now that there's Run?
 What's Megaparsec got that we ain't got?
 Add and benchmark naïve Seq
+chainl
+whitespace handling

bytes/bytes.go

@@ -0,0 +1,16 @@
+package bytes
+
+import (
+	"git.codemonkeysoftware.net/b/gigaparsec"
+)
+
+func Token[Out, WSOut any](whitespace gigaparsec.Parser[byte, WSOut]) func(p gigaparsec.Parser[byte, Out]) gigaparsec.Parser[byte, Out] {
+	mappedWS := gigaparsec.Map(whitespace, func(WSOut) struct{} { return struct{}{} })
+	var ignoreWS gigaparsec.Parser[byte, struct{}] = func(s gigaparsec.State[byte]) (gigaparsec.Result[byte, struct{}], error) {
+		result, err := mappedWS(s)
+		return result.Consume(false), err
+	}
+	return func(p gigaparsec.Parser[byte, Out]) gigaparsec.Parser[byte, Out] {
+		return gigaparsec.Seq2(p, gigaparsec.Repeat(0, ignoreWS), func(val Out, _ []struct{}) Out { return val })
+	}
+}

bytes/regexp.go

@@ -69,6 +69,9 @@ func Regexp(pattern string) gigaparsec.Parser[byte, string] {
 			return gigaparsec.Result[byte, string]{}, fmt.Errorf("Regexp: reader error: %w", err)
 		}
 		if idx == nil {
+			if err == io.EOF {
+				return gigaparsec.Fail[byte, string](false, gigaparsec.MessageEnd(input.Pos())), nil
+			}
 			got := make([]byte, r.Count())
 			_, _, err = input.Read(got)
 			if err != nil {
@@ -80,7 +83,7 @@ func Regexp(pattern string) gigaparsec.Parser[byte, string] {
 		// when searching a RuneReader.
 		dst := make([]byte, idx[1]-idx[0])
 		n, _, err := input.Read(dst)
-		if err != nil {
+		if err != nil && (!errors.Is(err, io.EOF) || n < uint64(len(dst))) {
 			return gigaparsec.Result[byte, string]{}, fmt.Errorf("Regexp: unexpected error: %w", err)
 		}
 		next := input.At(input.Pos() + n)

bytes/regexp_test.go

@@ -61,6 +61,21 @@ func TestRegexp(t *testing.T) {
 		must.NoError(t, err)
 		test.StrContains(t, result.Message().Got(), "hella")
 	})
+	t.Run("succeeds on empty matches", func(t *testing.T) {
+		p := pbytes.Regexp(".*")
+		result, err := p(gigaparsec.MakeState(strings.NewReader("")))
+		succeeded, value, _ := result.Status()
+		must.NoError(t, err)
+		must.True(t, succeeded)
+		must.EqOp(t, "", value)
+	})
+	t.Run("fails without an error at EOF", func(t *testing.T) {
+		p := pbytes.Regexp("a")
+		result, err := p(gigaparsec.MakeState(strings.NewReader("")))
+		succeeded, _, _ := result.Status()
+		must.NoError(t, err)
+		must.False(t, succeeded)
+	})
 }
 
 func TestRuneReader(t *testing.T) {

gigaparsec.go

@@ -194,6 +194,17 @@ func (p Parser[In, Out]) Label(label string) Parser[In, Out] {
 	}
 }
 
+func (p Parser[In, Out]) Where(pred func(Out) bool) Parser[In, Out] {
+	return func(s State[In]) (Result[In, Out], error) {
+		result, err := p(s)
+		if result.success && !pred(result.value) {
+			result.success = false
+			result.message.got = "failed Where predicate"
+		}
+		return result, err
+	}
+}
+
 type ParseError Message
 
 func (pe ParseError) Error() string {
@@ -403,3 +414,16 @@ func Repeat[In, Out any](minCount int, p Parser[In, Out]) Parser[In, []Out] {
 		}
 	}
 }
+
+// Lazy delays creating a parser from p until the parser is called.
+// This is useful for preventing recursive function calls in the
+// definition of a recursive parser.
+func Lazy[In, Out any](p func() Parser[In, Out]) Parser[In, Out] {
+	return func(s State[In]) (Result[In, Out], error) {
+		return p()(s)
+	}
+}
+
+func Bracket[In, Out, LOut, ROut any](left Parser[In, LOut], p Parser[In, Out], right Parser[In, ROut]) Parser[In, Out] {
+	return Seq3(left, p, right, func(_ LOut, val Out, _ ROut) Out { return val })
+}

parser_test.go

@@ -210,3 +210,11 @@ func TestRepeat(t *testing.T) {
 		test.False(t, succeeded)
 	})
 }
+
+func TestBracket(t *testing.T) {
+	Todo(t)
+}
+
+func TestWhere(t *testing.T) {
+	Todo(t)
+}