transformers- Concrete functor and monad transformers

Copyright(c) 2007 Yitzak Gale, Eric Kidd
LicenseBSD-style (see the file LICENSE)
Safe HaskellSafe




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.


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.




runMaybeT :: m (Maybe a)

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

Transform the computation inside a MaybeT.


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.