cats.effect._, HKTs, Kleisli, for comprehensions, and `implicit F: Monad[F]`; ADT promotion and more!

example.scala

package me.omega

import cats.effect.{IOApp, IO, ExitCode}
import cats.Monad
import cats.data.Kleisli
import cats.syntax.all._

object Main extends IOApp {
  override def run(args: List[String]): IO[ExitCode] = new App[IO].run.as(ExitCode.success) // `.as` is sugar for `.map[A](a: A)(_ => a)`
}

class App[F[_]](implicit F: Monad[F]) { // needed so that things of F[_] are flatMappable, so we can for-comp as in run()
  case class Thing(name: String)
  case class PromotedThing(name: String, exists: Boolean)
  
  // Kleislis compose easily; IO[String => Thing] and IO[String => PromotedThing] don't immediately.
  def run: F[Unit] = for {
    thingA <- createThing("foo").map(promoteThing(_)) // this will be Kleisli[IO, String, PromotedThing]
    thingB <- createThing("bar")                   // and this is Kleisli[IO, String, Thing]
  } yield doSomething(thingA, thingB) // pure function which operates on PromotedThing and Thing
  
  val createThing: Kleisli[F, String, Thing] =
    Kleisli((name: String) => F.pure(Thing(name))) // accept name: String and lift a new Thing into F[Thing]
  
  def promoteThing(thing: Thing): PromotedThing = PromotedThing(thing.name, true) // promote Thing to PromotedThing
}