base-4.9.1.0: 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: 4.9.0.0

Synopsis

- class Monad m => MonadFail m where

# Documentation

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: 4.9.0.0

Minimal complete definition

fail

Methods

fail :: String -> m a Source #

Instances

MonadFail [] # |
Methods fail :: String -> [a] Source #
MonadFail Maybe # |
Methods fail :: String -> Maybe a Source #
MonadFail IO # |
Methods fail :: String -> IO a Source #
MonadFail ReadP # |
Methods fail :: String -> ReadP a Source #
MonadFail ReadPrec # |
Methods fail :: String -> ReadPrec a Source #