fail package:fail

Forward-compatible MonadFail class This package contains the Control.Monad.Fail module providing the MonadFail class that became available in base-4.9.0.0 for older base package versions. This package turns into an empty package when used with GHC versions which already provide the Control.Monad.Fail module to make way for GHC's own Control.Monad.Fail module.

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.

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