| 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
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 { }
- 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
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.
Instances
Monad transformations
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.