Test Slice with a failing reader

TODO.txt

@@ -1,2 +1,4 @@
+Result should be Failed by default
 Test RuneReader
 Test Regexp
+Add SPDX tags

parser_test.go

@@ -2,10 +2,12 @@ package gigaparsec_test
 
 import (
 	"bytes"
+	"errors"
 	"testing"
 
 	"git.codemonkeysoftware.net/b/gigaparsec"
 	"git.codemonkeysoftware.net/b/gigaparsec/cursor"
+	ptest "git.codemonkeysoftware.net/b/gigaparsec/test"
 	"github.com/shoenig/test"
 	"github.com/shoenig/test/must"
 	"pgregory.net/rapid"
@@ -49,7 +51,14 @@ func TestSlice(t *testing.T) {
 		input := s[:inputLen]
 		assertParseFails(t, input, gigaparsec.Slice(s))
 	}))
-	t.Run("fails when read fails", Todo)
+	t.Run("fails when read fails", rapid.MakeCheck(func(t *rapid.T) {
+		expectedErr := rapid.Map(rapid.StringN(0, 100, -1), errors.New).Draw(t, "err")
+		r := ptest.ErrReaderAt(expectedErr)
+		c := cursor.NewReaderAt(r)
+		s := rapid.SliceOfN(rapid.Byte(), 0, 100).Draw(t, "s")
+		_, err := gigaparsec.Slice(s)(gigaparsec.MakeState(c))
+		test.ErrorIs(t, err, expectedErr)
+	}))
 	t.Run("succeeds when contents match", rapid.MakeCheck(func(t *rapid.T) {
 		input := rapid.SliceOfN(rapid.Byte(), 1, -1).Draw(t, "input")
 		sLen := rapid.IntRange(0, len(input)).Draw(t, "sLen")

test/readerat.go

@@ -0,0 +1,17 @@
+// Package test contains helpers for testing parsers.
+package test
+
+import "io"
+
+type errReaderAt struct {
+	err error
+}
+
+func (r errReaderAt) ReadAt([]byte, int64) (int, error) {
+	return 0, r.err
+}
+
+// ErrReaderAt returns an [io.ReaderAt] with a ReadAt method that always returns err.
+func ErrReaderAt(err error) io.ReaderAt {
+	return errReaderAt{err: err}
+}