Control.Monad.Fail (original) (raw)

Description

Transitional module providing the [MonadFail](Control-Monad-Fail.html#t:MonadFail "Control.Monad.Fail") class and primitive instances.

This module can be imported for defining forward compatible[MonadFail](Control-Monad-Fail.html#t:MonadFail "Control.Monad.Fail") instances:

import qualified Control.Monad.Fail as Fail

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

-- Provide legacy [fail](Control-Monad-Fail.html#v:fail "Control.Monad.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 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](Control-Monad.html#t:Monad "Control.Monad") without a [MonadFail](Control-Monad-Fail.html#t:MonadFail "Control.Monad.Fail") 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](Control-Monad-Fail.html#t:MonadFail "Control.Monad.Fail") should satisfy the following law: fail s should be a left zero for [>>=](Control-Monad.html#v:-62--62--61- "Control.Monad"),

fail s >>= f = fail s

If your [Monad](Control-Monad.html#t:Monad "Control.Monad") is also [MonadPlus](Control-Monad.html#v:MonadPlus "Control.Monad"), a popular definition is

fail _ = mzero

Since: base-4.9.0.0