Effects.kt

// Inspired by https://gist.github.com/tanmatra/3c2cec5d5d4345bea7a5f7c105af7238

import Effects.Companion.withEffects
import io.smallibs.utils.Await
import kotlinx.coroutines.GlobalScope
import kotlinx.coroutines.Job
import kotlinx.coroutines.launch
import kotlin.coroutines.Continuation
import kotlin.coroutines.resume
import kotlin.coroutines.suspendCoroutine
import kotlin.reflect.KClass

class Effects(var effects: List<Effect<*, *>>) {
    data class Effect<E : Any, O>(val klass: KClass<E>, val code: (E) -> (Continuation<O>) -> Unit) {
        fun apply(a: E, cont: Continuation<O>): Unit =
            code(a)(cont)
    }

    inline infix fun <reified E : Any, O> effect(noinline code: (E) -> (Continuation<O>) -> Unit): Effects {
        effects += Effect(E::class, code)
        return this
    }

    suspend fun <E : Any, O> perform(action: E): O = suspendCoroutine { cont ->
        for (anEffect in effects) {
            @Suppress("UNCHECKED_CAST")
            if (anEffect.klass.isInstance(action)) {
                val effect = anEffect as? Effect<E, O>
                if (effect != null) {
                    return@suspendCoroutine effect.apply(action, cont)
                }
            }
        }

        throw RuntimeException("No effect found") // TODO(It's also an effect)
    }

    infix fun handle(block: suspend Effects.() -> Any): Job =
        GlobalScope.launch { this@Effects.block() }

    companion object {
        fun withEffects(block: Effects.() -> Effects): Effects = Effects(listOf()).block()
    }
}

object Main {

    data class printString(val text: String)
    class readString()
    
    fun main() {
        withEffects {
            effect { p: printString ->
                { k: Continuation<Unit> ->
                    println(p.text)
                    k.resume(Unit)
                }
            }
            effect { p: readString ->
                { k: Continuation<String> ->
                    k.resume("Hello")
                }
            }
        } handle {
            val name: String = perform(readString())
            perform(printString("Hello $name"))
        }
    }
}