KleisliExample.scala

package sandbox

import cats.data.Kleisli
import cats.implicits._

case class User(username: String, avatarUrl: String, profileId: Int)
case class Avatar(url: String, monochrome: Boolean)
case class Profile(id: Int, age: Int)

object ComposingStuff extends App {
  val users: List[User] = List(
    User("dave", "http://dave.jpg", 1)
  )

  val avatars: List[Avatar] = List(
    // Avatar("http://dave.jpg", true)
  )

  val profiles: List[Profile] = List(
    // Profile(1, 42)
  )

  type Result[A] = Either[List[Throwable], A]

  val fetchUser: Kleisli[Result, String, User] =
    Kleisli { username =>
      println("fetching user " + username)

      val answer = users
        .find(_.username == username)
        .toRight(List(new Exception("user not found")))

      println("result of fetching user " + answer)

      answer
    }

  val fetchAvatar: Kleisli[Result, User, Avatar] =
    Kleisli { user =>
      println("fetching avatar " + user)

      val answer = avatars
        .find(_.url == user.avatarUrl)
        .toRight(List(new Exception("avatar not found")))

      println("result of fetching avatar " + answer)

      answer
    }

  val fetchProfile: Kleisli[Result, User, Profile] =
    Kleisli { user =>
      println("fetching profile " + user)

      val answer = profiles
        .find(_.id == user.profileId)
        .toRight(List(new Exception("profile not found")))

      println("result of fetching profile " + answer)

      answer
    }

  val fetchAvatarAndProfile: Kleisli[Result, User, (Avatar, Profile)] =
    (fetchAvatar, fetchProfile).parTupled

  val fetchAvatarAndProfileByUsername: Kleisli[Result, String, (Avatar, Profile)] =
    fetchUser.andThen(fetchAvatarAndProfile)

  println(fetchAvatarAndProfileByUsername.run("dave"))
}

// EXERCISE
//
// type RetrievalFunc[A, B] = A => Either[String, B]
// 
// case class IceCream(name: String, numCherries: Int, inCone: Boolean)
// 
// type OverallInput = List[String]
// type OverallOutput = IceCream
// 
// 1. ~Define Retrieval[A, B] as a Kleisli~
// 2. Write instances of Retrieval to:
//     - return the first, second, and third params in a list of string
//     - convert a string to an int
//     - convert a string to an boolean
// 3. Write an instance of Retrieval to:
//     - convert a list of strings to a IceCream