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://prime.haskell.org/wiki/Libraries/Proposals/MonadFail 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

*Since: base-4.9.0.0*