| 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 | Trustworthy |
| 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
Minimal complete definition