The problem with WriterT is it is too strict. See http://www.mail-archive.com/haskell@haskell.org/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.