Re: [Haskell-cafe] Re: Network parsing and parsec

20 Sep 2005


      You may like my parser transformer then (based on the efficent 
backtracking parser paper, I believe by Ralf Heinze - uses endofunctor 
and continuation passing - Its a long time since I tested it but I think 
it holds its own against Parsec, without requiring the extra return types).

-- parser.hs: Copyright (C)2001,2002 Keean Schupke.
--
--              Polymorphic monadic consumer based parser.

module Lib.Monad.ParserT(ParserT(..)) where

import Control.Monad hiding (guard)
import Control.Monad.Error
import Lib.Monad.MonadT
import Lib.Monad.MonadState
import Lib.Monad.MonadParser
import Lib.Monad.MonadControl
import Lib.Arrow.Runnable

------------------------------------------------------------------------------
-- An continuation passing endomorphic parser

type Cps a r = (a -> r) -> r
type Endo r = r -> r

newtype ParserT r tok m a = PT (Cps a ([tok] -> Endo (m r)))

instance Monad m => Functor (ParserT r tok m) where
        fmap g (PT m) = PT $ \k -> m (\a s f -> k (g a) s f)

instance Monad m => Monad (ParserT r tok m) where
        {-# INLINE return #-}
        return a = PT $ \k -> k a
        {-# INLINE (>>=) #-}
        (PT m) >>= f = PT $ \k -> m (\a -> (\(PT x) -> x) (f a) k)

instance Monad m => MonadPlus (ParserT r tok m) where
        {-# INLINE mzero #-}
        mzero = PT $ \_ _ f -> f
        {-# INLINE mplus #-}
        mplus (PT m) (PT n) = PT $ \k s -> m k s . n k s

instance MonadPlus m => MonadT (ParserT r tok) m where
        {-# INLINE up #-}
        up m = PT $ \k s f -> (m >>= \a -> k a s mzero) `mplus` f
        {-# INLINE down #-}
        down = undefined

instance (MonadPlus m,MonadT (ParserT r tok) m,Runnable ([tok] -> m 
([tok],r)) ([tok] -> n ([tok],r)))
                => Runnable (ParserT ([tok],r) tok m r) ([tok] -> n 
([tok],r)) where
        run = run . (\(PT m) t -> m (\a t' f -> return (t',a) `mplus` f) 
t mzero)

instance (MonadPlus m,MonadT (ParserT r tok) m)
                => Runnable (ParserT ([tok],r) tok m r) ([tok] -> m 
([tok],r)) where
        run = (\(PT m) t -> m (\a t' f -> return (t',a) `mplus` f) t mzero)

instance Monad m => MonadState [tok] (ParserT r tok m) where
        {-# INLINE update #-}
        update st = PT $ \k s -> k s ((st s) `asTypeOf` s)
        setState st = PT $ \k _ -> k () st
        getState = PT $ \k s -> k s s


instance Monad m => MonadParser tok (ParserT r tok m) where
        {-# INLINE item #-}
        item = PT $ \k s -> case s of
                [] -> id
                (a:x) -> k a x

instance (MonadPlus (t m),MonadParser tok m,MonadT t m) => MonadParser 
tok (t m) where
        item = up item

instance Monad m => MonadControl (ParserT r tok m) where
        {-# INLINE once #-}
        once (PT m) = PT $ \k s f -> m (\a s' _ -> k a s' f) s f

Regards,
    Keean.

John Goerzen wrote:
...
On 2005-09-16, Andrew Pimlott  wrote:
...
On Thu, Sep 15, 2005 at 06:11:58PM -0700, Andrew Pimlott wrote:
...
I don't see why this would be more error-prone than any other approach.
Hmm... I take that back.  I don't know anything about the IMAP protocol,
but after imagining for a few moments what it might be like, I can see
how it could be more difficult than my example.
The user state of the parser might help you...
Hmm, can you elaborate on that?
Basically, I *really* want to get away frmo having to use hGetContents.
It is just not at all friendly for an interactive netwrk protocol.  If I
were just streaming a large file from an FTP server, it would be fine,
but even using it to begin with involves using Handles in a nonstandard
way (since there must be a separate Handle for writing, since
hGetContents sents the Handle to be half-closed) that is apparently not
well-supported.
-- John
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