-----------------------------------------------------------------------------
The parser monad.

(c) 2001 Simon Marlow
-----------------------------------------------------------------------------

> module ParseMonad where

> data ParseResult a = OkP a | FailP String
> newtype P a = P (String -> Int -> ParseResult a)
> runP :: P a -> String -> Int -> ParseResult a
> runP (P f) = f

> lineP :: P Int
> lineP = P $ \_ l -> OkP l

> instance Monad P where
>       return m = P $ \ _ _ -> OkP m
>       m >>= k =  P $ \s l -> case runP m s l of
>               OkP a -> runP (k a) s l
>               FailP err -> FailP err
>       fail s = P $ \ _ _ -> FailP s