predicate logic

lib/Logic/Parse.hs

@@ -88,3 +88,6 @@ parseIf description check = Parser $ \input@(Input pos xs) ->
     (char:chars)
       | check char -> Right $ (char,) $ Input (pos + 1) chars
       | otherwise -> Left $ expected description input
+
+parseFail :: Show s => String -> Parser s a
+parseFail thing = Parser $ \input -> Left $ expected thing input