I'm trying to combine Twitter Future with Cats Kleisli and Arrow and I have a compilation error that I don't know how to solve it.
The code is the following:
package com.example
import scala.language.higherKinds
import cats.arrow.Arrow
import cats.implicits._
import cats.data.{EitherT, Kleisli}
import com.twitter.util.Future
object ArrowApp extends App {
type Resp[A] = EitherT[Future, Exception, A]
type KleisliResp[A] = Kleisli[Resp, List[Int], A]
val first: KleisliResp[Int] = Kleisli(_ => EitherT[Future, Exception, Int](Future.value(Right(1))))
val second: KleisliResp[String] = Kleisli(_ => EitherT[Future, Exception, String](Future.value(Right("TEST"))))
def combine[F[_, _] : Arrow, A, B, C](fab: F[A, B], fac: F[A, C]): F[A, (B, C)] = Arrow[F].lift((a: A) => (a, a)) >>> (fab *** fac)
val firstAndSecond: KleisliResp[(Int, String)] = combine(first, second)
}
The error that I get it is:
Error:(20, 31) could not find implicit value for evidence parameter of type cats.arrow.Arrow[[A, B]cats.data.Kleisli[com.example.cats.ArrowApp.Resp,A,B]]
val firstAndSecond: KleisliResp[(Int, String)] = combine(first, second)
If I replace the Twitter Future with Scala Future and I import the global executor import scala.concurrent.ExecutionContext.Implicits.global then code runs.
My build.sbt looks like:
organization := "com.example"
name := "scala-test"
version := "1.0"
scalaVersion := "2.12.3"
libraryDependencies ++= Seq(
"org.typelevel" %% "cats-core" % "1.2.0",
"org.typelevel" %% "cats-free" % "1.2.0",
"org.scala-lang.modules" %% "scala-parser-combinators" % "1.1.0",
"com.twitter" %% "finagle-core" % "18.3.0"
)
scalacOptions ++= Seq("-Ypartial-unification")
resolvers += Resolver.sonatypeRepo("releases")
addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.7")
Do you have any idea how to fix the compilation error?
Kind regards!
You probably need something like catbird, because it
[...] provides cats type class instances (and other useful cats-related stuff) for various Twitter Open Source Scala projects.
It currently includes the following:
- Type class instances for Future, Var, and Try (including Monad or MonadError, Semigroup, and equality)
Twitter's Future has nothing to do with scala's standard Future, and Cats does not provide any typeclasses for Twitter Futures by default.