我有下一个使用Cats IO编写的代码,它并行执行多个动作(简化):
import cats.effect._
import cats.implicits._
import scala.concurrent.ExecutionContext.Implicits.global
class ParallelExecIO {
def exec: IO[List[String]] = {
val foo = IO.shift *> IO("foo")
val bar = IO.shift *> IO("bar")
List(foo, bar).parSequence
}
}
是否可以使用效果抽象重写此代码?应提供哪些类型的证据?
样品:
class ParallelExecIO[F[_]: ConcurrentEffect /* ??? */] {
def exec: F[List[String]] = {
val foo = Async.shift[F](implicitly) *> "foo".pure[F]
val bar = Async.shift[F](implicitly) *> "bar".pure[F]
List(foo, bar).parSequence
}
}
[error] value parSequence不是List [F [String]]
的成员
答案 0 :(得分:3)
使用
scalaVersion := "2.12.5"
scalacOptions += "-Ypartial-unification"
libraryDependencies += "org.typelevel" %% "cats-core" % "1.1.0"
libraryDependencies += "org.typelevel" %% "cats-effect" % "0.10"
错误是
Error:(23, 22) could not find implicit value for parameter P: cats.Parallel[F,F]
List(foo, bar).parSequence
所以试试
import cats.Parallel
import cats.effect.{Async, ConcurrentEffect}
import cats.implicits._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.language.higherKinds
class ParallelExecIO[F[_]: ConcurrentEffect, G[_]](implicit ev: Parallel[F, G]) {
def exec: F[List[String]] = {
val foo = Async.shift[F](implicitly) *> "foo".pure[F]
val bar = Async.shift[F](implicitly) *> "bar".pure[F]
List(foo, bar).parSequence
}
}
当然你可以介绍 type Parallel[F[_]] = cats.Parallel[F, F]
,然后可以将其重写为class ParallelExecIO[F[_]: ConcurrentEffect : Parallel] {...