Fix predicate error cases

.gitignore

@@ -15,6 +15,10 @@
 # Test binary, built with `go test -c`
 *.test
 
+# Comparative benchmarks
+before.txt
+after.txt
+
 # Output of the go coverage tool, specifically when used with LiteIDE
 *.out
 coverage.html

parser.go

@@ -158,8 +158,7 @@ func Char(char rune) Parser[string] {
 //
 // If the predicate doesn't return false before the entire input is consumed, an error will be returned.
 //
-// A predicate that never returns true will leave the entire input unparsed and return a
-// value that is an empty string, and a remainder that is the entire input.
+// A predicate that never returns true will also return an error.
 func TakeWhile(predicate func(r rune) bool) Parser[string] {
 	return func(input string) (string, string, error) {
 		if input == "" {
@@ -174,6 +173,10 @@ func TakeWhile(predicate func(r rune) bool) Parser[string] {
 			return "", "", errors.New("TakeWhile: predicate must be a non-nil function")
 		}
 
+		if strings.IndexFunc(input, predicate) == -1 {
+			return "", "", errors.New("TakeWhile: predicate never returned true")
+		}
+
 		end := 0        // Byte position of last rune that the predicate returns true for
 		broken := false // Whether the predicate ever returned false so we broke the loop
 		for pos, char := range input {
@@ -200,11 +203,9 @@ func TakeWhile(predicate func(r rune) bool) Parser[string] {
 //
 // If the input is empty or predicate == nil, an error will be returned.
 //
-// If the predicate doesn't return true before the entire input is consumed, an error will be returned
-// .
+// If the predicate doesn't return true before the entire input is consumed, an error will be returned.
 //
-// A predicate that never returns false will leave the entire input unparsed and return a
-// value that is an empty string, and a remainder that is the entire input.
+// A predicate that never returns false will also return an error.
 func TakeUntil(predicate func(r rune) bool) Parser[string] {
 	return func(input string) (string, string, error) {
 		if input == "" {
@@ -219,6 +220,14 @@ func TakeUntil(predicate func(r rune) bool) Parser[string] {
 			return "", "", errors.New("TakeUntil: predicate must be a non-nil function")
 		}
 
+		// strings.IndexFunc returns the index of the first char for which the predicate
+		// returns true, we want the opposite to test if it ever returns false
+		notPred := func(r rune) bool { return !predicate(r) }
+
+		if strings.IndexFunc(input, notPred) == -1 {
+			return "", "", errors.New("TakeUntil: predicate never returned false")
+		}
+
 		end := 0        // Byte position of last rune that the predicate returns false for
 		broken := false // Whether the predicate ever returned true so we broke the loop
 		for pos, char := range input {

parser_test.go

@@ -584,7 +584,7 @@ func TestTakeWhile(t *testing.T) {
 			err:       "TakeWhile: predicate must be a non-nil function",
 		},
 		{
-			name:      "predicate never returns false", // Good libraries don't allow infinite loops
+			name:      "predicate never returns false",
 			input:     "fixed length input",
 			value:     "",
 			remainder: "",
@@ -596,10 +596,10 @@ func TestTakeWhile(t *testing.T) {
 			name:      "predicate never returns true",
 			input:     "fixed length input",
 			value:     "",
-			remainder: "fixed length input",
+			remainder: "",
 			predicate: func(r rune) bool { return false },
-			wantErr:   false,
-			err:       "",
+			wantErr:   true,
+			err:       "TakeWhile: predicate never returned true",
 		},
 		{
 			name:      "consume whitespace",
@@ -839,7 +839,7 @@ func TestTakeUntil(t *testing.T) {
 			err:       "TakeUntil: predicate must be a non-nil function",
 		},
 		{
-			name:      "predicate never returns true", // Good libraries don't allow infinite loops
+			name:      "predicate never returns true",
 			input:     "fixed length input",
 			value:     "",
 			remainder: "",
@@ -851,10 +851,10 @@ func TestTakeUntil(t *testing.T) {
 			name:      "predicate never returns false",
 			input:     "fixed length input",
 			value:     "",
-			remainder: "fixed length input",
+			remainder: "",
 			predicate: func(r rune) bool { return true },
-			wantErr:   false,
-			err:       "",
+			wantErr:   true,
+			err:       "TakeUntil: predicate never returned false",
 		},
 		{
 			name:      "consume until whitespace",