Polysemy.Internal.Effect

Documentation

class Effect e where

The class for semantic effects.

An effect e is a type e m a, where the other types are given by:

• The m type variable corresponds to a monad, which will eventually be instantiated at Sem---meaning it is capable of encoding arbitrary other effects.
• The a type is handled automatically and uninteresting.

The type e m must be a Functor, but this instance can always be given for free via the -XDeriveFunctor language extension. Often this instance must be derived as a standalone (-XStandaloneDeriving):

deriving instance Functor (MyEffect m)

If the effect doesn't use m whatsoever it is said to be first-order. First-order effects can be given an instance of Effect for free with -XDeriveAnyClass.

deriving instance Effect MyEffect

Minimal complete definition

Nothing

Methods

fmap' :: (a -> b) -> e m a -> e m b

Provide a specialised version of fmap to work around versions of GHC that cannot express the quantified constraint '(∀ m. Functor m => Functor (e m))'

This must always be equal to fmap, and can be removed once GHC 8.6 is the minimum version supported by this library

fmap' :: Functor (e m) => (a -> b) -> e m a -> e m b

Provide a specialised version of fmap to work around versions of GHC that cannot express the quantified constraint '(∀ m. Functor m => Functor (e m))'

This must always be equal to fmap, and can be removed once GHC 8.6 is the minimum version supported by this library

weave :: (Functor s, Functor m, Functor n) => s () -> (forall x. s (m x) -> n (s x)) -> (forall x. s x -> Maybe x) -> e m a -> e n (s a)

Higher-order effects require the ability to distribute state from other effects throughout themselves. This state is given by an initial piece of state s (), and a distributive law that describes how to move the state through an effect.

When the effect e has multiple computations in the m monad, weave defines the semantics for how these computations will view with the state:

• If the resulting state from one computation is fed to another, the second computation will see the state that results from the first computation.
• If instead it is given the intial state, both computations will see the same state, but the result of (at least) one will necessarily be ignored.

weave :: (Coercible (e m (s a)) (e n (s a)), Functor s, Functor m, Functor n) => s () -> (forall x. s (m x) -> n (s x)) -> (forall x. s x -> Maybe x) -> e m a -> e n (s a)

Higher-order effects require the ability to distribute state from other effects throughout themselves. This state is given by an initial piece of state s (), and a distributive law that describes how to move the state through an effect.

When the effect e has multiple computations in the m monad, weave defines the semantics for how these computations will view with the state:

• If the resulting state from one computation is fed to another, the second computation will see the state that results from the first computation.
• If instead it is given the intial state, both computations will see the same state, but the result of (at least) one will necessarily be ignored.

hoist :: (Functor m, Functor n) => (forall x. m x -> n x) -> e m a -> e n a

Lift a natural transformation from m to n over the effect. hoist should be defined as defaultHoist, but can be hand-written if the default performance isn't sufficient.

hoist :: (Coercible (e m a) (e n a), Functor m) => (forall x. m x -> n x) -> e m a -> e n a

Lift a natural transformation from m to n over the effect. hoist should be defined as defaultHoist, but can be hand-written if the default performance isn't sufficient.

Instances

Effect (Yo e)

Defined in Polysemy.Internal.Union

Methods

fmap' :: (a -> b) -> Yo e m a -> Yo e m b

weave :: (Functor s, Functor m, Functor n) => s () -> (forall x. s (m x) -> n (s x)) -> (forall x. s x -> Maybe x) -> Yo e m a -> Yo e n (s a)

hoist :: (Functor m, Functor n) => (forall x. m x -> n x) -> Yo e m a -> Yo e n a

Effect (Union r)

Defined in Polysemy.Internal.Union

Methods

fmap' :: (a -> b) -> Union r m a -> Union r m b

weave :: (Functor s, Functor m, Functor n) => s () -> (forall x. s (m x) -> n (s x)) -> (forall x. s x -> Maybe x) -> Union r m a -> Union r n (s a)

hoist :: (Functor m, Functor n) => (forall x. m x -> n x) -> Union r m a -> Union r n a

defaultHoist :: (Functor m, Functor n, Effect e) => (forall x. m x -> n x) -> e m a -> e n a

A default implementation of hoist. Particularly performance-sensitive effects should give a hand-written their own implementation of hoist.