Control.Monad.Fail

Transitional module providing the MonadFail class and primitive instances.

This module can be imported for defining forward compatible MonadFail instances:

import qualified Control.Monad.Fail as Fail

instance Monad Foo where
  (>>=) = {- ...bind impl... -}

  -- Provide legacy fail implementation for when
  -- new-style MonadFail desugaring is not enabled.
  fail = Fail.fail

instance Fail.MonadFail Foo where
  fail = {- ...fail implementation... -}

See https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail for more details.

Since: 4.9.0.0

Documentation

class Monad m => MonadFail m where

When a value is bound in do-notation, the pattern on the left hand side of <- might not match. In this case, this class provides a function to recover.

A Monad without a MonadFail instance may only be used in conjunction with pattern that always match, such as newtypes, tuples, data types with only a single data constructor, and irrefutable patterns (~pat).

Instances of MonadFail should satisfy the following law: fail s should be a left zero for >>=,

fail s >>= f  =  fail s

If your Monad is also MonadPlus, a popular definition is

fail _ = mzero

Since: 4.9.0.0

Minimal complete definition

fail

Methods

fail :: String -> m a

Instances

MonadFail []

Since: 4.9.0.0

Methods

fail :: String -> [a]

MonadFail Maybe

Since: 4.9.0.0

Methods

fail :: String -> Maybe a

MonadFail IO

Since: 4.9.0.0

Methods

fail :: String -> IO a

MonadFail ReadP

Since: 4.9.0.0

Methods

fail :: String -> ReadP a

MonadFail ReadPrec

Since: 4.9.0.0

Methods

fail :: String -> ReadPrec a

MonadFail (ST s)

Since: 4.10

Methods

fail :: String -> ST s a