Language.KURE.MonadCatch

This module provides classes for catch-like operations on Monads.

Monads with a Catch

class Monad m => MonadCatch m where

Monads with a catch for fail. The following laws are expected to hold:

fail msg `catchM` f == f msg
return a `catchM` f == return a

Methods

catchM :: m a -> (String -> m a) -> m a

Catch a failing monadic computation.

Instances

MonadCatch IO

The String is generated by showing the exception.

MonadCatch KureM

MonadCatch m => MonadCatch (OneR m)

MonadCatch m => MonadCatch (AnyR m)

MonadCatch m => MonadCatch (Transform c m a)

Lifting through a Reader transformer, where (c,a) is the read-only environment.

The KURE Monad

data KureM a

KureM is the minimal structure that can be an instance of MonadCatch. The KURE user is free to either use KureM or provide their own monad. KureM is essentially the same as Either String, except that it supports a MonadCatch instance which Either String does not (because its fail method calls error) A major advantage of this is that monadic pattern match failures are caught safely.

Instances

Monad KureM

Functor KureM

Applicative KureM

MonadCatch KureM

Eq a => Eq (KureM a)

Show a => Show (KureM a)

runKureM :: (a -> b) -> (String -> b) -> KureM a -> b

Eliminator for KureM.

fromKureM :: (String -> a) -> KureM a -> a

Get the value from a KureM, providing a function to handle the error case.

liftKureM :: Monad m => KureM a -> m a

Lift a KureM computation to any other monad.

The IO Monad

liftAndCatchIO :: (MonadCatch m, MonadIO m) => IO a -> m a

Lift a computation from the IO monad, catching failures in the target monad.

Combinators

(<+) :: MonadCatch m => m a -> m a -> m a infixl 3

A monadic catch that ignores the error message.

catchesM :: (Foldable f, MonadCatch m) => f (m a) -> m a

Select the first monadic computation that succeeds, discarding any thereafter.

tryM :: MonadCatch m => a -> m a -> m a

Catch a failing monadic computation, making it succeed with a constant value.

mtryM :: (MonadCatch m, Monoid a) => m a -> m a

Catch a failing monadic computation, making it succeed with mempty.

attemptM :: MonadCatch m => m a -> m (Either String a)

Catch a failing monadic computation, making it succeed with an error message.

testM :: MonadCatch m => m a -> m Bool

Determine if a monadic computation succeeds.

notM :: MonadCatch m => m a -> m ()

Fail if the monadic computation succeeds; succeed with () if it fails.

modFailMsg :: MonadCatch m => (String -> String) -> m a -> m a

Modify the error message of a failing monadic computation. Successful computations are unaffected.

setFailMsg :: MonadCatch m => String -> m a -> m a

Set the error message of a failing monadic computation. Successful computations are unaffected.

prefixFailMsg :: MonadCatch m => String -> m a -> m a

Add a prefix to the error message of a failing monadic computation. Successful computations are unaffected.

withPatFailMsg :: MonadCatch m => String -> m a -> m a

Use the given error message whenever a monadic pattern match failure occurs.