Control/Monad/Fail.hs

    {-# 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
    --
    module Control.Monad.Fail ( MonadFail(fail) ) where

    import GHC.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 4.9.0.0
    class Monad m => MonadFail m where
        fail :: String -> m a

    -- | @since 4.9.0.0
    instance MonadFail Maybe where
        fail _ = Nothing

    -- | @since 4.9.0.0
    instance MonadFail [] where
        {-# INLINE fail #-}
        fail _ = []

    -- | @since 4.9.0.0
    instance MonadFail IO where
        fail = failIO