interactors.kt

package com.exallium.librarytestbed

sealed class Interactor<out T> {
    class Tell<out T>(val msg: String, val t: T) : Interactor<T>() {
        override fun <B> flatMap(f: (T) -> B): Interactor<B> {
            return Tell(msg, f(t))
        }
    }

    class Ask<out T>(val k: (String) -> T) : Interactor<T>() {
        override fun <B> flatMap(f: (T) -> B): Interactor<B> {
            return Ask({ f(k(it)) })
        }
    }

    class ExitSuccess<out T> : Interactor<T>() {
        override fun <B> flatMap(f: (T) -> B): Interactor<B> {
            return ExitSuccess()
        }
    }

    abstract fun <B> flatMap(f: (T) -> B): Interactor<B>
}

class Free<out A : Interactor<T>, out T>(val a: A) {
    fun <B: Interactor<U>, U> flatMap(f: (A) -> Free<B, U>): Free<B, U> {
        return f(a)
    }
}

fun <T> putStrLn(msg: String, next: T) = Free(Interactor.Tell(msg, next))
fun <T> getLine(k: (String) -> T) = Free(Interactor.Ask(k))
fun exitSuccess() = Free(Interactor.ExitSuccess<Unit>())

fun <A : Interactor<T>, T> runOnConsole(free: Free<A, T>) {
    when (free.a) {
        is Interactor.Ask<*> -> {
            val input = "test input"
            runOnConsole(free.flatMap { free.a.k(input) as Free<Interactor<Any>, Any> })
        }
        is Interactor.Tell<*> -> {
            println(free.a.msg)
            runOnConsole(free.flatMap { free.a.t as Free<Interactor<Any>, Any> })
        }
        is Interactor.ExitSuccess<*> -> {
            //exitProcess(0)
        }
    }
}

val echo = putStrLn("hello", getLine({ putStrLn(it, exitSuccess()) }))

fun main(args: Array<String>) {
    runOnConsole(echo)
}