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 |
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://gitlab.haskell.org/haskell/prime/-/wikis/libraries/proposals/monad-fail for more details.
Since: base-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
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-4.9.0.0
Instances
MonadFail ReadP Source # | Since: base-4.9.0.0 |
MonadFail ReadPrec Source # | Since: base-4.9.0.0 |
MonadFail IO Source # | Since: base-4.9.0.0 |
MonadFail Maybe Source # | Since: base-4.9.0.0 |
MonadFail List Source # | Since: base-4.9.0.0 |
Defined in Control.Monad.Fail | |
MonadFail f => MonadFail (Ap f) Source # | Since: base-4.12.0.0 |