# Difference between revisions of "New monads/LazyWriterT"

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This came up on the mailing list: Why is WriterT never lazy? The answer is it does not use lazy patterns with "~". So here is a more useful NewMonads/LazyWriterT that add two "~" to the definition of (>>=) and renames WriterT to LazyWriterT. |
This came up on the mailing list: Why is WriterT never lazy? The answer is it does not use lazy patterns with "~". So here is a more useful NewMonads/LazyWriterT that add two "~" to the definition of (>>=) and renames WriterT to LazyWriterT. |
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## Latest revision as of 18:08, 26 October 2006

This came up on the mailing list: Why is WriterT never lazy? The answer is it does not use lazy patterns with "~". So here is a more useful NewMonads/LazyWriterT that add two "~" to the definition of (>>=) and renames WriterT to LazyWriterT.

This is very very handy when (Control.Monad.ST.Lazy) is the transformed Monad.

```
{-# OPTIONS_GHC -fglasgow-exts #-}
module Main where
-- LazyWriterT, copied from http://darcs.haskell.org/packages/mtl/Control/Monad/Writer.hs
import Control.Monad.Writer
import Control.Monad.Reader
import Control.Monad.Fix
import Control.Monad.Trans
newtype LazyWriterT w m a = LazyWriterT { runLazyWriterT :: m (a, w) }
instance (Monad m) => Functor (LazyWriterT w m) where
fmap f m = LazyWriterT $ do
(a, w) <- runLazyWriterT m
return (f a, w)
instance (Monoid w, Monad m) => Monad (LazyWriterT w m) where
return a = LazyWriterT $ return (a, mempty)
m >>= k = LazyWriterT $ do
~(a,w) <- runLazyWriterT m
~(b,w') <- runLazyWriterT (k a)
return (b, w `mappend` w')
fail msg = LazyWriterT $ fail msg
instance (Monoid w, MonadPlus m) => MonadPlus (LazyWriterT w m) where
mzero = LazyWriterT mzero
m `mplus` n = LazyWriterT $ runLazyWriterT m `mplus` runLazyWriterT n
instance (Monoid w, MonadFix m) => MonadFix (LazyWriterT w m) where
mfix m = LazyWriterT $ mfix $ \ ~(a, _) -> runLazyWriterT (m a)
instance (Monoid w, Monad m) => MonadWriter w (LazyWriterT w m) where
tell w = LazyWriterT $ return ((), w)
listen m = LazyWriterT $ do
(a, w) <- runLazyWriterT m
return ((a, w), w)
pass m = LazyWriterT $ do
((a, f), w) <- runLazyWriterT m
return (a, f w)
instance (Monoid w) => MonadTrans (LazyWriterT w) where
lift m = LazyWriterT $ do
a <- m
return (a, mempty)
instance (Monoid w, MonadIO m) => MonadIO (LazyWriterT w m) where
liftIO = lift . liftIO
instance (Monoid w, MonadReader r m) => MonadReader r (LazyWriterT w m) where
ask = lift ask
local f m = LazyWriterT $ local f (runLazyWriterT m)
execLazyWriterT :: Monad m => LazyWriterT w m a -> m w
execLazyWriterT m = do
(_, w) <- runLazyWriterT m
return w
mapLazyWriterT :: (m (a, w) -> n (b, w')) -> LazyWriterT w m a -> LazyWriterT w' n b
mapLazyWriterT f m = LazyWriterT $ f (runLazyWriterT m)
```