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

Language | Haskell2010 |

**Unless you absolutely have to use explicit lowering functions, use Polysemy.Final instead**

## Synopsis

- (.@!) :: (Monad base, Monad m) => base (forall x. Sem r x -> m x) -> ((forall x. Sem r x -> m x) -> base (forall y. Sem (e ': r) y -> Sem r y)) -> base (forall z. Sem (e ': r) z -> m z)
- nat :: Applicative base => (forall x. m x -> n x) -> base (forall x. m x -> n x)
- liftNat :: Applicative base => (forall x. (forall y. f y -> g y) -> m x -> n x) -> (forall y. f y -> g y) -> base (forall x. m x -> n x)
- fixedNat :: forall m n base. Applicative base => ((forall x. m x -> n x) -> forall x. m x -> n x) -> base (forall x. m x -> n x)
- (.@@!) :: (Monad base, Monad m) => base (forall x. Sem r x -> m x) -> ((forall x. Sem r x -> m x) -> base (forall y. Sem (e ': r) y -> Sem r (f y))) -> base (forall z. Sem (e ': r) z -> m (f z))
- nat' :: Applicative base => (forall x. m x -> n (f x)) -> base (forall x. m x -> n (f x))
- liftNat' :: Applicative base => (forall x. (forall y. f y -> g y) -> m x -> n (f x)) -> (forall y. f y -> g y) -> base (forall x. m x -> n (f x))
- fixedNat' :: forall m n f base. Applicative base => ((forall x. m x -> n (f x)) -> forall x. m x -> n (f x)) -> base (forall x. m x -> n (f x))

# Documentation

:: (Monad base, Monad m) | |

=> base (forall x. Sem r x -> m x) | The lowering function, likely |

-> ((forall x. Sem r x -> m x) -> base (forall y. Sem (e ': r) y -> Sem r y)) | |

-> base (forall z. Sem (e ': r) z -> m z) |

Like `.@`

, but useful for interpreters that wish to perform some
initialization before being run. Most of the time, you don't want to
duplicate this initialization every time your effect is lowered.

Consider an interpreter which wants to use an `IORef`

to store
intermediary state. It might begin like this:

myIntepreter ::`Member`

(`Lift`

`IO`

) r => (∀ x.`Sem`

r x ->`IO`

x) ->`Sem`

(MyEff ': r) a ->`Sem`

r a myInterpreter lower sem = do ref <-`sendM`

$`newIORef`

0 go ref sem where go ref =`interpretH`

$ e -> ...

This interpreter will do the wrong thing when composed via `.@`

. It
would have been correct if we didn't attempt to hide the creation of the
`IORef`

, but that's an unfortunate side-effect of wanting to hide
implementation details.

Instead, we can write `myInterpreter`

thusly:

myIntepreter :: (∀ x.`Sem`

r x ->`IO`

x) ->`IO`

(∀ a.`Sem`

(MyEff ': r) a ->`Sem`

r a) myInterpreter lower = do ref <-`newIORef`

0`nat`

$`interpretH`

$ e -> ...

and use `.@!`

(rather than `.@`

) to compose these things together.

Note: you must enable `-XImpredicativeTypes`

to give the correct type to
`myInterpreter`

here. Don't worry, it's (probably) not as scary as it
sounds.

*Since: 0.1.1.0*

nat :: Applicative base => (forall x. m x -> n x) -> base (forall x. m x -> n x) Source #

This is just `pure`

but with a type specialised for lifting interpreters.

*Since: 0.1.1.0*

liftNat :: Applicative base => (forall x. (forall y. f y -> g y) -> m x -> n x) -> (forall y. f y -> g y) -> base (forall x. m x -> n x) Source #

fixedNat :: forall m n base. Applicative base => ((forall x. m x -> n x) -> forall x. m x -> n x) -> base (forall x. m x -> n x) Source #

Like `nat`

, but for higher-order interpreters that need access to
themselves.

For example:

`fixedNat`

$ me ->`interpretH`

$ case SomeEffect -> ...

:: (Monad base, Monad m) | |

=> base (forall x. Sem r x -> m x) | The lowering function, likely |

-> ((forall x. Sem r x -> m x) -> base (forall y. Sem (e ': r) y -> Sem r (f y))) | |

-> base (forall z. Sem (e ': r) z -> m (f z)) |

Like `.@!`

, but for interpreters which change the resulting type --- eg.
'Polysemy.Error.lowerError.

*Since: 0.1.1.0*

nat' :: Applicative base => (forall x. m x -> n (f x)) -> base (forall x. m x -> n (f x)) Source #

liftNat' :: Applicative base => (forall x. (forall y. f y -> g y) -> m x -> n (f x)) -> (forall y. f y -> g y) -> base (forall x. m x -> n (f x)) Source #