Control.Monad.Trans.Abort

This module provides a monad and a monad transformer that allow the user to abort a monadic computation and immediately return a result.

The Abort monad

type Abort r = AbortT r Identity

An abort monad, parametrized by the type r of the value to return.

runAbort

Arguments

:: Abort r r

the monadic computation to run

-> r

the result of the computation

Execute the abort monad computation and return the resulting value.

The AbortT monad transformer

newtype AbortT r m a

An abort monad transformer parametrized by

• r - the value that will ultimately be returned; and
• m - the inner monad.

The AbortT type wraps a monadic value that is either

• Left r, which indicates that the monadic computation has terminated with result r and so all further steps in the computation should be ignored; or
• Right a, which indicates that the computation is proceding normally and that its current value is a.

Constructors

AbortT

Fields

• unwrapAbortT :: m (Either r a)

Instances

MonadTrans (AbortT r)

Defined in Control.Monad.Trans.Abort

Methods

lift :: Monad m => m a -> AbortT r m a

Monad m => Monad (AbortT r m)

Defined in Control.Monad.Trans.Abort

Methods

(>>=) :: AbortT r m a -> (a -> AbortT r m b) -> AbortT r m b

(>>) :: AbortT r m a -> AbortT r m b -> AbortT r m b

return :: a -> AbortT r m a

fail :: String -> AbortT r m a

Functor m => Functor (AbortT r m)

Defined in Control.Monad.Trans.Abort

Methods

fmap :: (a -> b) -> AbortT r m a -> AbortT r m b

(<$) :: a -> AbortT r m b -> AbortT r m a

Applicative m => Applicative (AbortT r m)

Defined in Control.Monad.Trans.Abort

Methods

pure :: a -> AbortT r m a

(<*>) :: AbortT r m (a -> b) -> AbortT r m a -> AbortT r m b

liftA2 :: (a -> b -> c) -> AbortT r m a -> AbortT r m b -> AbortT r m c

(*>) :: AbortT r m a -> AbortT r m b -> AbortT r m b

(<*) :: AbortT r m a -> AbortT r m b -> AbortT r m a

MonadIO m => MonadIO (AbortT r m)

Defined in Control.Monad.Trans.Abort

Methods

liftIO :: IO a -> AbortT r m a

runAbortT

Arguments

:: Monad m

=> AbortT r m r

the monadic computation to run

-> m r

the (monadic) result of the computation

Execute the abort monad computation and return the resulting (monadic) value.

Abort operations

abort

Arguments

:: Monad m

=> r

the result to return

-> AbortT r m a

a monadic value that has the effect of terminating the computation and immediately returning a value; note that since all subsequent steps in the computation will be ignored, this monadic value can take an arbitrary type since its value will never be accessed

Abort the computation and immediately return a result; all steps in the computation after this monadic computation will be ignored.

Note that since no further computation is performed after this, there is no way for subsequent computations to access the monadic value, and so it can be assigned an arbitrary type.

lifters

liftCallCC

Arguments

:: (((Either r a -> m (Either r b)) -> m (Either r a)) -> m (Either r a))

callCC on the argument monad.

-> ((a -> AbortT r m b) -> AbortT r m a)

AbortT action that receives the continuation

-> AbortT r m a

Lifts a callCC operation to AbortT.

liftCatch

Arguments

:: (m (Either r a) -> (e -> m (Either r a)) -> m (Either r a))

catch on the argument monad.

-> AbortT r m a

AbortT action to attempt.

-> (e -> AbortT r m a)

Exception handler.

-> AbortT r m a

Lift a catchError operation to AbortT.

liftListen

Arguments

:: Monad m

=> (m (Either r a) -> m (Either r a, w))

listen on the argument monad.

-> AbortT r m a

AbortT action to run.

-> AbortT r m (a, w)

Lift a listen operation to the new monad.

liftPass

Arguments

:: Monad m

=> (m (Either r a, w -> w) -> m (Either r a))

pass on the argument monad.

-> AbortT r m (a, w -> w)

AbortT action to run.

-> AbortT r m a

Lift a pass operation to the new monad.