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Cont.scala
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Cont.scala
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/*
* Copyright 2020-2022 Typelevel
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package cats.effect.kernel
import cats.~>
/**
* This construction supports `Async.cont`
* {{{
* trait Async[F[_]] {
* ...
*
* def cont[A](body: Cont[F, A]): F[A]
* }
* }}}
* It's a low level operation meant for implementors, end users should use `async`, `start` or
* `Deferred` instead, depending on the use case.
*
* It can be understood as providing an operation to resume an `F` asynchronously, of type
* `Either[Throwable, A] => Unit`, and an (interruptible) operation to semantically block until
* resumption, of type `F[A]`. We will refer to the former as `resume`, and the latter as `get`.
*
* These two operations capture the essence of fiber blocking, and can be used to build `async`,
* which in turn can be used to build `Fiber`, `start`, `Deferred` and so on.
*
* Refer to the default implementation to `Async[F].async` for an example of usage.
*
* The reason for the shape of the `Cont` construction in `Async[F].cont`, as opposed to simply:
*
* {{{
* trait Async[F[_]] {
* ...
*
* def cont[A]: F[(Either[Throwable, A] => Unit, F[A])]
* }
* }}}
*
* is that it's not safe to use concurrent operations such as `get.start`.
*
* The `Cont` encoding therefore simulates higher-rank polymorphism to ensure that you can not
* call `start` on `get`, but only use operations up to `MonadCancel` (`flatMap`, `onCancel`,
* `uncancelable`, etc).
*
* If you are an implementor, and you have an implementation of `async` but not `cont`, you can
* override `Async[F].async` with your implementation, and use `Async.defaultCont` to implement
* `Async[F].cont`.
*/
trait Cont[F[_], K, R] extends Serializable {
def apply[G[_]](
implicit
G: MonadCancel[G, Throwable]): (Either[Throwable, K] => Unit, G[K], F ~> G) => G[R]
}