{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-}
{- |
Copyright : 2007 Eric Kidd
License : BSD3
Stability : experimental
Portability : non-portable (multi-parameter type classes, undecidable instances)
The 'MaybeT' monad. See
for more widely-used
version. Our 'Functor' instance and our implementation of '>>=' are
borrowed from there.
[Computation type:] Computations which may fail or return nothing.
[Binding strategy:] Failure returns the value 'Nothing', bypassing any
bound functions which follow. Success returns a value wrapped in 'Just'.
[Useful for:] Building computations from steps which may fail. No error
information is returned. (If error information is required, see
'Control.Monad.Error'.)
-}
module Control.Monad.Maybe (
MaybeT(..)
-- * Limitations
-- $Limitations
-- * Example
-- $MaybeExample
) where
import Control.Monad()
import Control.Monad.Trans()
import Control.Monad.Cont
import Control.Monad.Fix()
import Control.Monad.Reader
import Control.Monad.State
import Control.Monad.Writer
-- | A monad transformer which adds Maybe semantics to an existing monad.
newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }
instance (Functor m) => Functor (MaybeT m) where
fmap f = MaybeT . fmap (fmap f) . runMaybeT
instance (Monad m) => Monad (MaybeT m) where
fail _ = MaybeT (return Nothing)
return = lift . return
x >>= f = MaybeT (runMaybeT x >>= maybe (return Nothing) (runMaybeT . f))
instance (Monad m) => MonadPlus (MaybeT m) where
mzero = MaybeT (return Nothing)
mplus x y = MaybeT $ do v <- runMaybeT x
case v of
Nothing -> runMaybeT y
Just _ -> return v
instance MonadTrans MaybeT where
lift x = MaybeT (liftM Just x)
instance (MonadCont m) => MonadCont (MaybeT m) where
-- Again, I hope this is correct.
callCC f = MaybeT (callCC (\c -> runMaybeT (f (wrap c))))
where wrap :: (Maybe a -> m (Maybe b)) -> a -> MaybeT m b
wrap c = MaybeT . c . Just
-- MonadError: MonadError has fairly weird semantics when lifted by MaybeT,
-- so let's skip it for now.
instance (MonadIO m) => MonadIO (MaybeT m) where
liftIO = lift . liftIO
instance (MonadFix m) => MonadFix (MaybeT m) where
-- I hope this is correct. At a minimum, it typechecks.
mfix f = MaybeT (mfix (maybe (return Nothing) (runMaybeT . f)))
-- MonadList: Not implemented.
-- MonadPlus: Ambiguous. See note in introduction.
-- Requires -fallow-undecidable-instances.
instance (MonadReader r m) => MonadReader r (MaybeT m) where
ask = lift ask
local f m = MaybeT (local f (runMaybeT m))
-- MonadRWS: Not implemented.
-- Taken from http://www.haskell.org/haskellwiki/New_monads/MaybeT .
instance MonadState s m => MonadState s (MaybeT m) where
get = lift get
put = lift . put
-- Requires -fallow-undecidable-instances.
instance (MonadWriter w m) => MonadWriter w (MaybeT m) where
tell = lift . tell
listen m = MaybeT (listen (runMaybeT m) >>= (return . liftMaybe))
where liftMaybe (Nothing, _) = Nothing
liftMaybe (Just x, w) = Just (x,w)
-- I'm not sure this is useful, but it's the best I can do:
pass m = MaybeT (runMaybeT m >>= maybe (return Nothing)
(liftM Just . pass . return))
{- $Limitations
The instance @MonadPlus@ is not provided, because it has ambiguous
semantics. It could refer to either
>instance MonadPlus m => MonadPlus (MaybeT m)
...lifting the semantics of an underlying 'MaybeT' monad, or
>instance MonadPlus (MaybeT m)
...with semantics similar to @MonadPlus Maybe@.
-}
{- $MaybeExample
Here is an example that shows how to use 'MaybeT' to propagate an
end-of-file condition in the IO monad. In the example below, both
@maybeReadLine@ and @failIfQuit@ may cause a failure, which will propagate
out to @main@ without further intervention.
>import System.Console.Readline
>import Data.Maybe
>import Control.Monad
>import Control.Monad.Trans
>import Control.Monad.Maybe
>
>-- 'MaybeIO' is the type of computations which do IO, and which may fail.
>type MaybeIO = MaybeT IO
>
>-- 'readline' already has type 'String -> IO (Maybe String)'; we just need
>-- to wrap it.
>maybeReadLine :: String -> MaybeIO String
>maybeReadLine prompt = MaybeT (readline prompt)
>
>-- Fail if 'str' equals "quit".
>failIfQuit :: (Monad m) => String -> m ()
>failIfQuit str = when (str == "quit") (fail "Quitting")
>
>-- This task may fail in several places. Try typing Control-D or "quit" at
>-- any prompt.
>concatTwoInputs :: MaybeIO ()
>concatTwoInputs = do
> s1 <- maybeReadLine "String 1> "
> failIfQuit s1
> s2 <- maybeReadLine "String 2> "
> failIfQuit s2
> liftIO (putStrLn ("Concatenated: " ++ s1 ++ s2))
>
>-- Loop until failure.
>main :: IO ()
>main = do
> result <- runMaybeT concatTwoInputs
> if isNothing result
> then putStrLn "Bye!"
> else main
-}