Sorry to bother again. I just cannot figure out how it could compile. I got compile errors. Can someone point out what is right code to use a do notion to make a Parser works. Thanks in advance. -------------------------------------------------------------------------------------------- newtype Parser a = P { parse :: (String -> [(a,String)]) } instance Monad Parser where return v = P (\s -> [(v,s)]) p >>= f = P (\s -> case parse p s of [] -> [] [(v,str)] -> parse (f v) str) fail _ = P (\_ -> []) item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] p :: Parser (Char,Char) p = do x <- item item y <- item return (x,y) ---------------------------------------------------------------------------------