Control.Monad.Fail

Copyright	(C) 2015 David Luposchainsky (C) 2015 Herbert Valerio Riedel
License	BSD-style (see the file LICENSE)
Maintainer	[email protected]
Stability	provisional
Portability	portable
Safe Haskell	Trustworthy
Language	Haskell2010

Description

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: base-4.9.0.0

class Monad m => MonadFail m where Source

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: base-4.9.0.0

Methods

fail :: String -> m a Source

Instances

Instances details

MonadFail []	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> [a] Source
MonadFail Maybe	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> Maybe a Source
MonadFail IO	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> IO a Source
MonadFail ReadP	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadP Methods fail :: String -> ReadP a Source
MonadFail ReadPrec	Since: base-4.9.0.0
Instance details Defined in Text.ParserCombinators.ReadPrec Methods fail :: String -> ReadPrec a Source
MonadFail (ST s)	Since: base-4.11.0.0
Instance details Defined in GHC.ST Methods fail :: String -> ST s a Source
MonadFail (ST s)	Since: base-4.10
Instance details Defined in Control.Monad.ST.Lazy.Imp Methods fail :: String -> ST s a Source
MonadFail f => MonadFail (Ap f)	Since: base-4.12.0.0
Instance details Defined in Data.Monoid Methods fail :: String -> Ap f a Source

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.10.2/docs/html/libraries/base-4.14.1.0/Control-Monad-Fail.html