Adjust the effect stack by a contravariant transformation function over the stack. This function reveals the profunctorial nature of Eff; in particular, Eff is a profunctor [Effect] -> Type, lmap is adjust, and rmap is fmap.

Lift a computation into a bigger effect stack with one more effect. For a more general version see raiseN.

Lift a computation into a bigger effect stack with arbitrarily more effects. This function requires TypeApplications.

Lift a computation with a fixed, known effect stack into some superset of the stack.

Eliminate a duplicate effect from the top of the effect stack. For a more general version see subsumeN.

Eliminate several duplicate effects from the top of the effect stack. This function requires TypeApplications.

Like raise, but adds the new effect under the top effect. This is useful for transforming an interpreter e' :> es => Eff (e : es) ~> Eff es into a reinterpreter Eff (e : es) ~> Eff (e' : es):

myInterpreter :: Bar :> es => Eff (Foo : es) ~> Eff es
myInterpreter = ...

myReinterpreter :: Eff (Foo : es) ~> Eff (Bar : es)
myReinterpreter = myInterpreter . raiseUnder

In other words,

reinterpret h == interpret h . raiseUnder

However, note that this function is suited for transforming an existing interpreter into a reinterpreter; if you want to define a reinterpreter from scratch, you should still prefer reinterpret, which is both easier to use and more efficient.

Since: 0.2.0.0

Like raiseUnder, but allows introducing multiple effects. This function requires TypeApplications.

Since: 0.2.0.0

Like raiseUnder, but allows introducing the effect under multiple effects. This function requires TypeApplications.

Since: 0.2.0.0

A generalization of both raiseUnderN and raiseNUnder, allowing introducing multiple effects under multiple effects. This function requires TypeApplications and is subject to serious type ambiguity; you most likely will need to supply all three type variables explicitly.

Since: 0.2.0.0

The typeclass that denotes a handler scope, handling effect e sent from the effect stack esSend in the effect stack es.

You should not define instances for this typeclass whatsoever.

Get the send-site Env.

The type of an effect handler, which is a function that transforms an effect e from an arbitrary effect stack into computations in the effect stack es.

The type of a simple transformation function from effect e to e'.

Interpret an effect e in terms of effects in the effect stack es with an effect handler.

Like interpret, but adds a new effect e' to the stack that can be used in the handler.

Like reinterpret, but adds two new effects.

Like reinterpret, but adds three new effects.

Like reinterpret, but adds arbitrarily many new effects. This function requires TypeApplications.

Respond to an effect, but does not eliminate it from the stack. This means you can re-send the operations in the effect handler; it is often useful when you need to "intercept" operations so you can add extra behaviors like logging.

Like interpose, but allows to introduce one new effect to use in the handler.

Like impose, but allows introducing arbitrarily many effects. This requires TypeApplications.

Interpret an effect in terms of another effect in the stack via a simple Translator.

transform trans = interpret (sendVia toEff . trans)

Like transform, but instead of using an effect in stack, add a new one to the top of it.

translate trans = reinterpret (sendVia toEff . trans)

Run a computation in the current effect stack; this is useful for interpreting higher-order effects. For example, if you want to interpret a bracketing effects in terms of IO:

data Resource m a where
  Bracket :: m a -> (a -> m ()) -> (a -> m b) -> Resource m b

You will not be able to simply write this for the effect:

runBracket :: IOE :> es => Eff (Resource : es) a -> Eff es a
runBracket = interpret \case
  Bracket alloc dealloc use -> UnliftIO.bracket alloc dealloc use

This is because effects are sended from all kinds of stacks that has Resource in it, so effect handlers received the effect as Resource esSend a, where esSend is an arbitrary stack with Resource, instead of Resource es a. This means alloc, dealloc and use are of type Eff esSend a, while bracket can only take and return Eff es a. So we need to use toEff, which converts an Eff esSend a into an Eff es a:

runBracket :: IOE :> es => Eff (Resource : es) a -> Eff es a
runBracket = interpret \case
  Bracket alloc dealloc use -> UnliftIO.bracket
    (toEff alloc)
    (toEff . dealloc)
    (toEff . use)

Run a computation in the current effect stack, just like toEff, but takes a Handler of the current effect being interpreted, so that inside the computation being ran, the effect is interpreted differently. This is useful for interpreting effects with local contexts, like Local:

runReader :: r -> Eff (Reader r : es) ~> Eff es
runReader x = interpret (handle x)
  where
    handle :: r -> Handler (Reader r) es
    handle r = \case
      Ask       -> pure r
      Local f m -> toEffWith (handle $ f r) m

Temporarily gain the ability to lift some Eff es actions into Eff esSend. This is only useful for dealing with effect operations with the monad type in the negative position, which means it's unlikely that you need to use this function in implementing your effects.