{-# LANGUAGE Trustworthy #-}{-# LANGUAGE NoImplicitPrelude #-}-- |-- Module : GHC.Internal.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--moduleGHC.Internal.Control.Monad.Fail (MonadFail (fail ))whereimportGHC.Internal.Base (String ,Monad (),Maybe (Nothing ),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 'Control.Monad.>>=',---- @-- fail s >>= f = fail s-- @---- If your 'Monad' is also 'Control.Monad.MonadPlus', a popular definition is---- @-- fail _ = mzero-- @---- @fail s@ should be an action that runs in the monad itself, not an-- exception (except in instances of @MonadIO@). In particular,-- @fail@ should not be implemented in terms of @error@.---- @since base-4.9.0.0classMonad m =>MonadFail m wherefail ::String ->m a -- | @since base-4.9.0.0instanceMonadFail Maybe wherefail :: forall a. String -> Maybe a
fail String
_=Maybe a
forall a. Maybe a
Nothing -- | @since base-4.9.0.0instanceMonadFail []where{-# INLINEfail #-}fail :: forall a. String -> [a]
fail String
_=[]-- | @since base-4.9.0.0instanceMonadFail IO wherefail :: forall a. String -> IO a
fail =String -> IO a
forall a. String -> IO a
failIO