| 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 |
| Safe Haskell | Safe |
| Language | Haskell2010 |
Control.Monad.Fail
Description
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
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 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