Polysemy.Cont

Effect

data Cont ref m a where

An effect for abortive continuations.

Formulated à la Tom Schrijvers et al. "Monad Transformers and Modular Algebraic Effects: What Binds Them Together" (2016). http://www.cs.kuleuven.be/publicaties/rapporten/cw/CW699.pdf

Activating polysemy-plugin is highly recommended when using this effect in order to avoid ambiguous types.

Constructors

Jump :: ref a -> a -> Cont ref m b

Subst :: (ref a -> m b) -> (a -> m b) -> Cont ref m b

Instances

type DefiningModule (Cont :: (Type -> Type) -> (k -> Type) -> k -> Type)

Defined in Polysemy.Cont.Internal

type DefiningModule (Cont :: (Type -> Type) -> (k -> Type) -> k -> Type) = "Polysemy.Cont.Internal"

Actions

jump :: forall ref a b r. Member (Cont ref) r => ref a -> a -> Sem r b

Provide an answer to a prompt, jumping to its reified continuation, and aborting the current continuation.

Using jump will rollback all effectful state back to the point where the prompt was created, unless such state is interpreted in terms of the final monad, or the associated interpreter of the effectful state is run after runContUnsafe, which may be done if the effect isn't higher-order.

Higher-order effects do not interact with the continuation in any meaningful way; i.e. local or censor does not affect it, and catch will fail to catch any of its exceptions. The only exception to this is if you interpret such effects and Cont in terms of the final monad, and the final monad can perform such interactions in a meaningful manner.

subst :: forall ref a b r. Member (Cont ref) r => (ref a -> Sem r b) -> (a -> Sem r b) -> Sem r b

Reifies the current continuation in the form of a prompt, and passes it to the first argument. If the prompt becomes invoked via jump, then the second argument will be run before the reified continuation, and otherwise will not be called at all.

callCC :: forall ref a r. Member (Cont ref) r => ((forall b. a -> Sem r b) -> Sem r a) -> Sem r a

Call with current continuation. Executing the provided continuation will abort execution.

Using the provided continuation will rollback all effectful state back to the point where callCC was invoked, unless such state is interpreted in terms of the final monad, or the associated interpreter of the effectful state is run after runContUnsafe, which may be done if the effect isn't higher-order.

Higher-order effects do not interact with the continuation in any meaningful way; i.e. local or censor does not affect it, and catch will fail to catch any of its exceptions. The only exception to this is if you interpret such effects and Cont in terms of the final monad, and the final monad can perform such interactions in a meaningful manner.

Interpretations

runContPure :: Sem '[Cont (Ref (Sem '[]) a)] a -> Sem '[] a

Runs a Cont effect by providing pure as the final continuation.

This is a safe variant of runContUnsafe, as this may only be used as the final interpreter before run.

runContM :: Sem '[Cont (Ref (Sem '[Embed m]) a), Embed m] a -> Sem '[Embed m] a

Runs a Cont effect by providing pure as the final continuation.

This is a safe variant of runContUnsafe, as this may only be used as the final interpreter before runM.

runContFinal :: (Member (Final m) r, MonadCont m) => Sem (Cont (ExitRef m) ': r) a -> Sem r a

Runs a Cont effect in terms of a final MonadCont

Beware: Effects that aren't interpreted in terms of the final monad will have local state semantics in regards to Cont effects interpreted this way. See interpretFinal.

Unsafe Interpretations

runContUnsafe :: Sem (Cont (Ref (Sem r) a) ': r) a -> Sem r a

Runs a Cont effect by providing pure as the final continuation.

Beware: This interpreter will invalidate all higher-order effects of any interpreter run after it; i.e. local and censor will be no-ops, catch will fail to catch exceptions, and listen will always return mempty.

__You should therefore use runContUnsafe after running all interpreters for your higher-order effects.__

Note that Final is a higher-order effect, and thus runContUnsafe can't safely be used together with runFinal.

Prompt types

newtype Ref m s a

Constructors

Ref

Fields

• runRef :: a -> m s

newtype ExitRef m a

Constructors

ExitRef

Fields

• enterExit :: forall b. a -> m b