Safe Haskell | None |
---|---|

Language | Haskell2010 |

## Synopsis

- data Optional s m a where
- Optionally :: s a -> m a -> Optional s m a

- threadRegionalViaOptional :: (ThreadsEff t (Optional (Const s)), Monad m) => (forall x. Regional s m x -> m x) -> Regional s (t m) a -> t m a

# Effects

data Optional s m a where Source #

A *helper primitive effect* for manipulating a region, with the option
to execute it in full or in part. `s`

is expected to be a functor.

Helper primitive effects are effects that allow you to avoid interpreting one
of your own effects as a primitive if the power needed from direct access to
the underlying monad can instead be provided by the relevant helper primitive
effect. The reason why you'd want to do this is that helper primitive effects
already have `ThreadsEff`

instances defined for them, so you don't have to
define any for your own effect.

The helper primitive effects offered in this library are -- in order of
ascending power -- `Regional`

,
`Optional`

, `BaseControl`

and `Unlift`

.

The typical use-case of `Optional`

is to lift a natural transformation
of a base monad equipped with the power to recover from an exception.
`HoistOption`

and accompanying interpreters is
provided as a specialization of `Optional`

for this purpose.

`Optional`

in its most general form lacks a pre-defined interpreter:
when not using `HoistOption`

, you're expected to
define your own interpreter for `Optional`

(treating it as a primitive effect).

** Optional is typically used as a primitive effect.**
If you define a

`Carrier`

that relies on a novel
non-trivial monad transformer `t`

, then you need to make
a `Functor s => ``ThreadsEff`

t (`Optional`

s)

instance (if possible).
`threadOptionalViaBaseControl`

can help you with that.The following threading constraints accept `Optional`

:

Optionally :: s a -> m a -> Optional s m a |

#### Instances

Carrier m => PrimHandler HoistOptionH (HoistOption m) m Source # | |

Defined in Control.Effect.Internal.Optional effPrimHandler :: EffPrimHandler (HoistOption m) m Source # | |

Functor s => ThreadsEff ListT (Optional s) Source # | |

Functor s => ThreadsEff (ExceptT e) (Optional s) Source # | |

(Functor s, Monoid w) => ThreadsEff (WriterT w) (Optional s) Source # | |

Functor s => ThreadsEff (StateT s') (Optional s) Source # | |

ThreadsEff (ReaderT i) (Optional s) Source # | |

Functor s => ThreadsEff (StateT s') (Optional s) Source # | |

(Functor s, Monoid w) => ThreadsEff (WriterT w) (Optional s) Source # | |

(Functor s, Monoid w) => ThreadsEff (WriterT w) (Optional s) Source # | |

Functor s => ThreadsEff (FreeT f) (Optional s) Source # | |

# Threading utilities

threadRegionalViaOptional :: (ThreadsEff t (Optional (Const s)), Monad m) => (forall x. Regional s m x -> m x) -> Regional s (t m) a -> t m a Source #

A valid definition of `threadEff`

for a

instance,
given that `ThreadsEff`

(`Regional`

s) t`t`

threads

for any functor `Optional`

f`f`

.