Control.Monad.Fail

base-4.12.0.0: Basic libraries

Copyright | (C) 2015 David Luposchainsky (C) 2015 Herbert Valerio Riedel
License | BSD-style (see the file LICENSE)
Maintainer | libraries@haskell.org
Stability | provisional
Portability | portable
Safe Haskell | Trustworthy
Language | Haskell2010

Control.Monad.Fail

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

Synopsis

- class Monad m => MonadFail m where
  - fail :: String -> m a

# 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: base-4.9.0.0

Methods

fail :: String -> m a #

Instances

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