Control.Effect.Lift

Given a Lift n constraint in a signature carried by m, sendM promotes arbitrary actions of type n a to m a. It is spiritually similar to lift from the MonadTrans typeclass.

A type-restricted variant of sendM for IO actions.

This is particularly useful when you have a Has (Lift IO) sig m constraint for the use of liftWith, and want to run an action abstracted over MonadIO. IO has a MonadIO instance, and sendIO’s type restricts the action’s type to IO without further type annotations.

Run actions in an outer context.

This can be used to provide interoperation with base functionality like Control.Exception.catch:

liftWith $ \ hdl ctx -> catch (hdl (m <$ ctx)) (hdl . (<$ ctx) . h)

The higher-order function takes both an initial context, and a handler phrased as a distributive law (as described in the documentation for Handler). This handler takes actions lifted into a context functor, which can be either the initial context, or the derived context produced by handling a previous action.

As with MonadBaseControl, care must be taken when lifting functions like Control.Exception.finally which don’t use the return value of one of their actions, as this can lead to dropped effects.

The class of carriers (results) for algebras (effect handlers) over signatures (effects), whose actions are given by the alg method.

m is a carrier for sig containing eff.

Note that if eff is a sum, it will be decomposed into multiple Member constraints. While this technically allows one to combine multiple unrelated effects into a single Has constraint, doing so has two significant drawbacks:

1. Due to a problem with recursive type families, this can lead to significantly slower compiles.

2. It defeats ghc’s warnings for redundant constraints, and thus can lead to a proliferation of redundant constraints as code is changed.

Run an action exhausted of effects to produce its final result value.