Tagless with Arrow & typeclassless using extension functions and instances

typeclassless_tagless_extensions.kt

import arrow.Kind
import arrow.core.Option
import arrow.core.Try
import arrow.core.functor
import arrow.effects.IO
import arrow.effects.fix
import arrow.effects.functor
import arrow.typeclasses.Functor

/* algebras */
interface Service1<F> : Functor<F> {
  fun Kind<F, Int>.addOne(): Kind<F, Int> =
    map { it + 1 }
}

interface Service2<F> : Functor<F> {
  fun Kind<F, Int>.addTwo(): Kind<F, Int> =
    map { it + 2 }
}

interface App<F> : Service1<F>, Service2<F> {
  fun Kind<F, Int>.addThree(): Kind<F, Int> =
    addOne().addTwo()
}

/* Our ap works for all functors */
val <F> Functor<F>.app: App<F>
  get() = object : App<F> {
    override fun <A, B> Kind<F, A>.map(f: (A) -> B): Kind<F, B> = this@app.run { map(f) }
  }

/** End of the world **/
object test {
  @JvmStatic
  fun main(args: Array<String>) {
    Option.functor().app.run { Option(1).addThree() }.let(::println) //Some(4)
    Try.functor().app.run { Try { 1 }.addThree() }.let(::println) //Success(value=4)
    IO.functor().app.run { IO { 1 }.addThree().fix().unsafeRunSync() }.let(::println) //4
  }
}