tagless-demo.scala

import cats.Monad
import cats.effect.{IO, Sync}
import cats.syntax.flatMap._
import cats.syntax.functor._

trait Command[F[_]] {
  def getStrLn: F[String]
  def putStrLn(string: String): F[Unit]
}

class CommandProgram[F[_]: Monad](implicit C: Command[F]) {
  def get2StrLns: F[String] =
    C.getStrLn.flatMap(a => C.getStrLn.map(b => a + "\n" + b))
}

class CommandInterpreter[F[_]](implicit F: Sync[F]) extends Command[F] {
  override def getStrLn: F[String] =
    F.delay { scala.io.StdIn.readLine }

  override def putStrLn(string: String): F[Unit] =
    F.delay { println(string) }
}

class OtherProgram[F[_]](implicit C: CommandProgram[F]) {
  def someLogic: F[String] = C.get2StrLns
}

class SomeProgram[F[_]: Monad](implicit C: Command[F]) {
  def someLogic: F[Unit] = C.getStrLn.flatMap(C.putStrLn)
}

implicit val cmd = new CommandInterpreter[IO]
implicit val cmdExt: CommandProgram[IO] = new CommandProgram[IO]

println(new OtherProgram[IO].someLogic.unsafeRunSync())