# Difference between revisions of "New monads/MaybeT"

From HaskellWiki

(Added <haskell>) |
(Import/export of Fix and State not needed yet.) |
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Line 9: | Line 9: | ||

(MaybeT(runMaybeT), |
(MaybeT(runMaybeT), |
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module Control.Monad, |
module Control.Monad, |
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− | module Control.Monad.Fix, |
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module Control.Monad.Trans) |
module Control.Monad.Trans) |
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where |
where |
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import Control.Monad |
import Control.Monad |
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− | import Control.Monad.Fix |
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import Control.Monad.Trans |
import Control.Monad.Trans |
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− | import Control.Monad.State |
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newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)} |
newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)} |

## Revision as of 00:11, 26 February 2007

The Maybe monad deserves a transformer, just like the other classic monads.

## The code

```
{-# OPTIONS_GHC -fglasgow-exts #-}
module Control.Monad.Maybe
(MaybeT(runMaybeT),
module Control.Monad,
module Control.Monad.Trans)
where
import Control.Monad
import Control.Monad.Trans
newtype MaybeT m a = MaybeT {runMaybeT :: m (Maybe a)}
instance Functor m => Functor (MaybeT m) where
fmap f x = MaybeT $ fmap (fmap f) $ runMaybeT x
instance Monad m => Monad (MaybeT m) where
return = MaybeT . return . return
x >>= f = MaybeT $ runMaybeT x >>= maybe (return Nothing) (runMaybeT . f)
fail _ = MaybeT $ return Nothing
instance Monad m => MonadPlus (MaybeT m) where
mzero = MaybeT $ return mzero
mplus x y = MaybeT $ liftM2 mplus (runMaybeT x) (runMaybeT y)
-- Provide other MTL instances, for convenience
instance MonadTrans MaybeT where
lift = MaybeT . liftM return
-- (Add other MTL instances, and a MonadFix instance)
```