[Haskell-cafe] style question: Writer monad or unsafeIOToST?

Gregory Wright gwright at comcast.net
Thu Aug 24 15:14:16 EDT 2006


Hi Chris,

Thank you.  That is exactly what I needed to know.

It's good to know that I'm not totally crazy and that with the
lazier LogT the code can run as it was written.  It seems
as if a request should be made for a Writer.Lazy as well as
the existing Writer.Strict.  (The latter could well be the default,
just as with the ST monad.)  A good idea?

Virtual beer to you sir!

-Greg

On Aug 24, 2006, at 1:05 PM, Chris Kuklewicz wrote:

> The problem with WriterT is it is too strict.
>
> See http://www.mail-archive.com/[email protected]/msg16088.html
>
> The fix is adding ~ to the patterns inside the definition of (>>=):
>
> 		~(a,w)  <- runLogT m
> 		~(b,w') <- runLogT (k a)
>
> A lazy version of WriterT, called LogT:
>
>> {-# OPTIONS_GHC -fglasgow-exts #-}
>> module Main where
>> import Control.Monad.ST.Lazy
>> import Data.STRef.Lazy
>> import Control.Monad.Writer
>> import Control.Monad.Identity
>> import Control.Monad.Fix
>> import Control.Monad.Trans
>> import Control.Monad.Reader
>> import Maybe
>> import Debug.Trace
>> type LogMonoid = [String] -> [String]
>> loopLT :: Int -> LogT [String] Identity [Int]
>> loopLT 0 = trace "end of loopLT" (return [0])
>> loopLT x = do
>>   let msg = "loopLT now "++ show x
>>   tell [msg]
>>   liftM (x:) (loopLT (pred x))
>> newtype LogT w m a = LogT { runLogT :: m (a, w) }
>> instance (Monad m) => Functor (LogT w m) where
>> 	fmap f m = LogT $ do
>> 		(a, w) <- runLogT m
>> 		return (f a, w)
>> instance (Monoid w, Monad m) => Monad (LogT w m) where
>> 	return a = LogT $ return (a, mempty)
>> 	m >>= k  = LogT $ do
>> 		~(a,w)  <- runLogT m
>> 		~(b,w') <- runLogT (k a)
>> 		return (b, w `mappend` w')
>> 	fail msg = LogT $ fail msg
>> instance (Monoid w, MonadPlus m) => MonadPlus (LogT w m) where
>> 	mzero       = LogT mzero
>> 	m `mplus` n = LogT $ runLogT m `mplus` runLogT n
>> instance (Monoid w, MonadFix m) => MonadFix (LogT w m) where
>> 	mfix m = LogT $ mfix $ \ ~(a, _) -> runLogT (m a)
>> instance (Monoid w, Monad m) => MonadWriter w (LogT w m) where
>> 	tell   w = LogT $ return ((), w)
>> 	listen m = LogT $ do
>> 		(a, w) <- runLogT m
>> 		return ((a, w), w)
>> 	pass   m = LogT $ do
>> 		((a, f), w) <- runLogT m
>> 		return (a, f w)
>> instance (Monoid w) => MonadTrans (LogT w) where
>> 	lift m = LogT $ do
>> 		a <- m
>> 		return (a, mempty)
>> instance (Monoid w, MonadIO m) => MonadIO (LogT w m) where
>> 	liftIO = lift . liftIO
>> -- This instance needs -fallow-undecidable-instances, because --  
>> it does not satisfy the coverage condition
>> instance (Monoid w, MonadReader r m) => MonadReader r (LogT w m)  
>> where
>> 	ask       = lift ask
>> 	local f m = LogT $ local f (runLogT m)
>> execLogT :: Monad m => LogT w m a -> m w
>> execLogT m = do
>> 	(_, w) <- runLogT m
>> 	return w
>> mapLogT :: (m (a, w) -> n (b, w')) -> LogT w m a -> LogT w' n b
>> mapLogT f m = LogT $ f (runLogT m)
>> main :: IO ()
>> main = do
>>   let logLT = runIdentity (execLogT (loopLT 100))
>>   print (head logLT)
>>   print (last logLT)
>
> The output is
>
>  ./maindemo
> "loopLT now 100"
> end of loopLT
> "loopLT now 1"
>
> Just as we want.
>
>



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