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{-# LANGUAGE Trustworthy #-}{-# LANGUAGE NoImplicitPrelude #-}-- |-- Module      :  Control.Monad.Fail-- 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---- 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--moduleControl.Monad.Fail(MonadFail(fail))whereimportGHC.Base(String,Monad(),Maybe(Nothing),IO())import{-# SOURCE#-}GHC.IO(failIO)-- | 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 4.9.0.0classMonadm=>MonadFailmwherefail::String->ma-- | @since 4.9.0.0instanceMonadFailMaybewherefail_=Nothing-- | @since 4.9.0.0instanceMonadFail[]where{-# INLINEfail#-}fail_=[]-- | @since 4.9.0.0instanceMonadFailIOwherefail=failIO