# Difference between revisions of "New monads/MaybeT"

From HaskellWiki

m (Missing [ for Category:Code) |
(Added <haskell>) |
||

Line 3: | Line 3: | ||

== The code == |
== The code == |
||

+ | <haskell> |
||

{-# OPTIONS_GHC -fglasgow-exts #-} |
{-# OPTIONS_GHC -fglasgow-exts #-} |
||

Line 37: | Line 38: | ||

-- (Add other MTL instances, and a MonadFix instance) |
-- (Add other MTL instances, and a MonadFix instance) |
||

+ | </haskell> |
||

[[Category:Code]] |
[[Category:Code]] |

## Revision as of 11:38, 5 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.Fix,
module Control.Monad.Trans)
where
import Control.Monad
import Control.Monad.Fix
import Control.Monad.Trans
import Control.Monad.State
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)
```