Simple example of cats-mtl library

cats-mtl-1.scala

/*
val CatsMTLVersion = "0.4.0"
val CatsVersion = "1.4.0"
val CatsEffectVersion = "1.0.0"

libraryDependencies ++= Seq(
  "org.typelevel" %% "cats-core" % CatsVersion,
  "org.typelevel" %% "cats-effect" % CatsEffectVersion,
  "org.typelevel" %% "cats-mtl-core" % CatsMTLVersion,
*/

import cats._
import cats.data._
import cats.mtl._
import cats.implicits._
import cats.mtl.implicits._
import cats.effect.IO

object MTLFetch {

  case class Cat(name: String, yearOfBirth: Int)
  val sampleCat1 = Cat("Garfield", 1965)
  val sampleCat2 = Cat("Winky", 1973)

  type Log = Vector[String]

  def program[F[_] : Monad](implicit ft : FunctorTell[F, Log],
    aa: ApplicativeAsk[F, Cat]) : F[Int] = {

    for(
      name <- aa.ask.map(cat => cat.name);
      age <- aa.ask.map(cat => cat.yearOfBirth);
      _ <- ft.tell(Vector("I did it!"));
      _ <- ft.tell(Vector("You did what?"));
      _ <- ft.tell(Vector(s"Found the cat called ${name}'s age is $age"))
    ) yield (2018 - age)
  }

  def main(args: Array[String]): Unit = {

    // Materialize the program with monad transformer instances

    val w = program[ReaderT[WriterT[Id, Log, ?], Cat, ?]].run(sampleCat1)

    val (log, result) = w.run

    println(result, log)

    // (53,Vector(I did it!, You did what?, Found the cat called Garfield's age is 1965))

  }
}