semigroupの素振り

cats_semigroup_exercise.scala

import cats.implicits._
import cats.{Semigroup, Show}
import cats.effect.{Effect, IO}

sealed trait MessageProcessResult
object MessageProcessResult {

  case object Done extends MessageProcessResult
  case object Pending extends MessageProcessResult
  val values: Seq[MessageProcessResult] = Seq(Done, Pending)

}

implicit val show:Show[MessageProcessResult] = Show {
  case MessageProcessResult.Done => "Done"
  case MessageProcessResult.Pending => "Pending"
}

implicit val sg:Semigroup[MessageProcessResult] = (x: MessageProcessResult, y: MessageProcessResult) => (x, y) match {
  case (MessageProcessResult.Done, MessageProcessResult.Done) => MessageProcessResult.Done
  case _ => MessageProcessResult.Pending
}

import MessageProcessResult._
import scala.language.higherKinds

def foldResult[F[_]:Effect](fa: F[MessageProcessResult], fb: F[MessageProcessResult], fc:F[MessageProcessResult]): F[MessageProcessResult] = for {
  a <- fa
  b <- fb
  c <- fc
} yield a |+| b |+| c

type F[A] = IO[A]

foldResult[F](
  Effect[F].pure(Done),
  Effect[F].pure(Pending),
  Effect[F].pure(Done)
).unsafeRunSync()
//res0: MessageProcessResult = Pending