tagless.scala

Welcome to the Ammonite Repl 0.8.2
(Scala 2.12.1 Java 1.8.0_121)
@ import $ivy.`org.typelevel::cats:0.9.0`, cats._, cats.implicits._
import $ivy.$                          , cats._, cats.implicits._
@
@ trait Maths[F[_]] {
    def int(i: Int): F[Int]
    def add(l: F[Int], r: F[Int]): F[Int]
  }
defined trait Maths
@
@ trait Logging[F[_]] {
    def info(s: String): F[Unit]
  }
defined trait Logging
@
@ class MyProgram[F[_]](alg: Maths[F] with Logging[F])(implicit M: Monad[F]) {
    import alg._
    def addTwoNumbers(a: Int, b: Int): F[Int] =
      for {
        _ <- info(s"Adding $a and $b")
        result <- add(int(a), int(b))
      } yield result
  }
defined class MyProgram
@
@ val interp = new Maths[Id] with Logging[Id] {
    def int(a:Int) = a
    def add(a: Id[Int], b: Id[Int]) = a + b
    def info(msg: String) = println(msg)
  }
interp: AnyRef with Maths[Id] with Logging[Id] = $sess.cmd4$$anon$1@60ce75d0
@
@ val prog = new MyProgram[Id](interp)
prog: MyProgram[Id] = $sess.cmd3$MyProgram@27353d1c
@
@ prog.addTwoNumbers(3, 4)
Adding 3 and 4
res6: Id[Int] = 7

tagless2.scala

Welcome to the Ammonite Repl 0.8.2
(Scala 2.12.1 Java 1.8.0_121)
@ import $ivy.`org.typelevel::cats:0.9.0`, cats._, cats.implicits._
import $ivy.$                          , cats._, cats.implicits._
@
@ trait Maths[F[_]] {
    def int(i: Int): F[Int]
    def add(l: F[Int], r: F[Int]): F[Int]
  }
defined trait Maths
@
@ trait Logging[F[_]] {
    def info(s: String): F[Unit]
  }
defined trait Logging
@
@ object MathsStuff {
    // don't even need the Monad bound in this case, but you get the idea
    def addTwoNumbers[F[_]: Monad](a: Int, b: Int)(implicit alg: Maths[F]): F[Int] = {
      import alg._
      add(int(a), int(b))
    }
  }
defined object MathsStuff
@
@ object LogStuff {
    def writeSomeThings[F[_]: Monad](implicit alg: Logging[F]): F[Unit] = {
      import alg._
      for {
        _ <- info("hello")
        _ <- info("world")
      } yield ()
    }
  }
defined object LogStuff
@
@ object MyProg {
    def addAndLog[F[_]: Monad](implicit alg: Maths[F] with Logging[F]): F[Int] = {
      for {
        _ <- LogStuff.writeSomeThings[F]
        result <- MathsStuff.addTwoNumbers[F](3, 4)
      } yield result
    }
  }
defined object MyProg
@
@ implicit val interp = new Maths[Id] with Logging[Id] {
    def int(a:Int) = a
    def add(a: Id[Int], b: Id[Int]) = a + b
    def info(msg: String) = println(msg)
  }
interp: AnyRef with Maths[Id] with Logging[Id] = $sess.cmd7$$anon$1@550e427a
@
@ MyProg.addAndLog[Id]
hello
world
res8: Id[Int] = 7