Polysemy.IdempotentLowering

Documentation

(.@!) infixl 8

Arguments

:: (Monad base, Monad m)

=> base (forall x. Sem r x -> m x)

The lowering function, likely nat runM.

-> ((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)

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)

Lift a combinator designed to be used with .@ into one designed to be used with .@!.

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)

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

For example:

fixedNat $ me -> interpretH $ case
  SomeEffect -> ...

(.@@!) infixl 8

Arguments

:: (Monad base, Monad m)

=> base (forall x. Sem r x -> m x)

The lowering function, likely nat runM.

-> ((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))

nat' is to nat as .@@! is to .@!.

Since: 0.1.1.0

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))

Lift a combinator designed to be used with .@@ into one designed to be used with .@@!.

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))

fixedNat' is to fixedNat as nat' is to nat.