transformers-0.5.2.0: Concrete functor and monad transformers

Copyright | (c) 2007 Yitzak Gale Eric Kidd
License | BSD-style (see the file LICENSE)
Maintainer | R.Paterson@city.ac.uk
Stability | experimental
Portability | portable
Safe Haskell | Safe
Language | Haskell98

Control.Monad.Trans.Maybe

Contents

- The MaybeT monad transformer
- Conversion
- Lifting other operations

Description

The `MaybeT` monad transformer extends a monad with the ability to exit the computation without returning a value.

A sequence of actions produces a value only if all the actions in the sequence do. If one exits, the rest of the sequence is skipped and the composite action exits.

For a variant allowing a range of exception values, see Control.Monad.Trans.Except.

Synopsis

- newtype MaybeT m a = MaybeT {
  - runMaybeT :: m (Maybe a)
  }
- mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
- maybeToExceptT :: Functor m => e -> MaybeT m a -> ExceptT e m a
- exceptToMaybeT :: Functor m => ExceptT e m a -> MaybeT m a
- liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b
- liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a
- liftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a
- liftPass :: Monad m => Pass w m (Maybe a) -> Pass w (MaybeT m) a

# The MaybeT monad transformer

newtype MaybeT m a Source #

The parameterizable maybe monad, obtained by composing an arbitrary monad with the `Maybe` monad.

Computations are actions that may produce a value or exit.

The `return` function yields a computation that produces that value, while `>>=` sequences two subcomputations, exiting if either computation does.

Constructors

MaybeT |
Fields runMaybeT :: m (Maybe a)

Instances

MonadTrans MaybeT # |
Methods lift :: Monad m => m a -> MaybeT m a Source #
Monad m => Monad (MaybeT m) # |
Methods (>>=) :: MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b Source # (>>) :: MaybeT m a -> MaybeT m b -> MaybeT m b Source # return :: a -> MaybeT m a Source # fail :: String -> MaybeT m a Source #
Functor m => Functor (MaybeT m) # |
Methods fmap :: (a -> b) -> MaybeT m a -> MaybeT m b Source # (<$) :: a -> MaybeT m b -> MaybeT m a Source #
MonadFix m => MonadFix (MaybeT m) # |
Methods mfix :: (a -> MaybeT m a) -> MaybeT m a Source #
Monad m => MonadFail (MaybeT m) # |
Methods fail :: String -> MaybeT m a Source #
(Functor m, Monad m) => Applicative (MaybeT m) # |
Methods pure :: a -> MaybeT m a Source # (<*>) :: MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b Source # (*>) :: MaybeT m a -> MaybeT m b -> MaybeT m b Source # (<*) :: MaybeT m a -> MaybeT m b -> MaybeT m a Source #
Foldable f => Foldable (MaybeT f) # |
Methods fold :: Monoid m => MaybeT f m -> m Source # foldMap :: Monoid m => (a -> m) -> MaybeT f a -> m Source # foldr :: (a -> b -> b) -> b -> MaybeT f a -> b Source # foldr' :: (a -> b -> b) -> b -> MaybeT f a -> b Source # foldl :: (b -> a -> b) -> b -> MaybeT f a -> b Source # foldl' :: (b -> a -> b) -> b -> MaybeT f a -> b Source # foldr1 :: (a -> a -> a) -> MaybeT f a -> a Source # foldl1 :: (a -> a -> a) -> MaybeT f a -> a Source # toList :: MaybeT f a -> [a] Source # null :: MaybeT f a -> Bool Source # length :: MaybeT f a -> Int Source # elem :: Eq a => a -> MaybeT f a -> Bool Source # maximum :: Ord a => MaybeT f a -> a Source # minimum :: Ord a => MaybeT f a -> a Source # sum :: Num a => MaybeT f a -> a Source # product :: Num a => MaybeT f a -> a Source #
Traversable f => Traversable (MaybeT f) # |
Methods traverse :: Applicative f => (a -> f b) -> MaybeT f a -> f (MaybeT f b) Source # sequenceA :: Applicative f => MaybeT f (f a) -> f (MaybeT f a) Source # mapM :: Monad m => (a -> m b) -> MaybeT f a -> m (MaybeT f b) Source # sequence :: Monad m => MaybeT f (m a) -> m (MaybeT f a) Source #
Eq1 m => Eq1 (MaybeT m) # |
Methods liftEq :: (a -> b -> Bool) -> MaybeT m a -> MaybeT m b -> Bool Source #
Ord1 m => Ord1 (MaybeT m) # |
Methods liftCompare :: (a -> b -> Ordering) -> MaybeT m a -> MaybeT m b -> Ordering Source #
Read1 m => Read1 (MaybeT m) # |
Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (MaybeT m a) Source # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [MaybeT m a] Source #
Show1 m => Show1 (MaybeT m) # |
Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> MaybeT m a -> ShowS Source # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [MaybeT m a] -> ShowS Source #
MonadZip m => MonadZip (MaybeT m) # |
Methods mzip :: MaybeT m a -> MaybeT m b -> MaybeT m (a, b) Source # mzipWith :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c Source # munzip :: MaybeT m (a, b) -> (MaybeT m a, MaybeT m b) Source #
MonadIO m => MonadIO (MaybeT m) # |
Methods liftIO :: IO a -> MaybeT m a Source #
(Functor m, Monad m) => Alternative (MaybeT m) # |
Methods empty :: MaybeT m a Source # (<|>) :: MaybeT m a -> MaybeT m a -> MaybeT m a Source # some :: MaybeT m a -> MaybeT m [a] Source # many :: MaybeT m a -> MaybeT m [a] Source #
Monad m => MonadPlus (MaybeT m) # |
Methods mzero :: MaybeT m a Source # mplus :: MaybeT m a -> MaybeT m a -> MaybeT m a Source #
(Eq1 m, Eq a) => Eq (MaybeT m a) # |
Methods (==) :: MaybeT m a -> MaybeT m a -> Bool # (/=) :: MaybeT m a -> MaybeT m a -> Bool #
(Ord1 m, Ord a) => Ord (MaybeT m a) # |
Methods compare :: MaybeT m a -> MaybeT m a -> Ordering # (<) :: MaybeT m a -> MaybeT m a -> Bool # (<=) :: MaybeT m a -> MaybeT m a -> Bool # (>) :: MaybeT m a -> MaybeT m a -> Bool # (>=) :: MaybeT m a -> MaybeT m a -> Bool # max :: MaybeT m a -> MaybeT m a -> MaybeT m a # min :: MaybeT m a -> MaybeT m a -> MaybeT m a #
(Read1 m, Read a) => Read (MaybeT m a) # |
Methods readsPrec :: Int -> ReadS (MaybeT m a) Source # readList :: ReadS [MaybeT m a] Source # readPrec :: ReadPrec (MaybeT m a) Source # readListPrec :: ReadPrec [MaybeT m a] Source #
(Show1 m, Show a) => Show (MaybeT m a) # |
Methods showsPrec :: Int -> MaybeT m a -> ShowS Source # show :: MaybeT m a -> String Source # showList :: [MaybeT m a] -> ShowS Source #

mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b Source #

Transform the computation inside a `MaybeT`.

-     runMaybeT (mapMaybeT f m) = f (runMaybeT m)

# Conversion

maybeToExceptT :: Functor m => e -> MaybeT m a -> ExceptT e m a Source #

Convert a `MaybeT` computation to `ExceptT`, with a default exception value.

exceptToMaybeT :: Functor m => ExceptT e m a -> MaybeT m a Source #

Convert a `ExceptT` computation to `MaybeT`, discarding the value of any exception.

# Lifting other operations

liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b Source #

Lift a `callCC` operation to the new monad.

liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a Source #

Lift a `catchE` operation to the new monad.

liftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a Source #

Lift a `listen` operation to the new monad.

liftPass :: Monad m => Pass w m (Maybe a) -> Pass w (MaybeT m) a Source #

Lift a `pass` operation to the new monad.