ZIO prelude 의 Reader Monad

문제

zpure 를 이용한 Reader Monad 를 살펴보자

코드

import zio._
import zio.prelude.Reader

case class Cat(name: String)
case class Dog(name: String)

object ZpureReaderTest {
  def main(args: Array[String]): Unit = {
    val catName: Reader[Cat, String] = Reader.serviceWith(_.name)
    val dogName: Reader[Dog, String] = Reader.serviceWith(_.name)

    val allName: Reader[Dog with Cat, String] = for {
      c <- catName
      d <- dogName
    } yield s"All names: cat=$c, dog=$d"

    val dog = Dog("Puppy")
    val cat = Cat("Kitty")

    val env: ZEnvironment[Dog with Cat] = ZEnvironment[Dog, Cat](dog, cat)
    val r = allName.provideEnvironment(env).run
    println(r) // All names: cat=Kitty, dog=Puppy
  }
}

설명

Error 까지 포함 시킬 경우 아래처럼 EReader 를 사용할 수 있다.

import zio.prelude.Reader

val catName: EReader[Cat, String, Nothing] = ZPure.serviceWithPure[Cat](cat => ZPure.fail("FAIL !!!"))