StateExample.kt

import kategory.*

typealias Stack = ListKW<Int>

/* Normal function */
fun pop(stack: Stack): Either<String, Tuple2<Stack, Int>> =
        if (stack.isNotEmpty()) (stack.drop(1).k() toT stack.first()).right()
        else "Stack is empty".left()

/* Normal function */
fun push(a: Int, stack: Stack): Stack = (listOf(a) + stack).k()

/* wrapped as state */
fun pushState(a: Int) = StateT<EitherKindPartial<String>, Stack, Unit>(run = { stack: Stack ->
    (push(a, stack) toT Unit).right()
})

/* alternative wrapped as state */
fun pushState2(a: Int) = StateT.modify<EitherKindPartial<String>, Stack> { push(a, it).right() }

fun stackManip() = StateT.monad<EitherKindPartial<String>, Stack>().binding {
    pushState(3).bind()
    ::pop.stateT().bind()
    val i = ::pop.stateT().bind()
    yields(i)
}.ev()

fun stackyStack() = State.monad<Stack>().binding {
    val stackNow = State.get<Stack>().bind()
    val r = (if (stackNow == listOf(1, 2, 3).k()) State.put(listOf(8, 3, 1).k())
    else State.put(listOf(9, 2, 1).k())).bind()
    yields(r)
}.ev()

fun main(args: Array<String>) {
    stackManip().runM(listOf(5, 8, 2, 1).k()).let(::println)
    stackManip().runM(listOf<Int>().k()).let(::println)
    stackyStack().run(listOf(1, 2, 3).k()).let(::println)
}

/**
 * Pop written in monad binding style
 */
fun pop2() = State.monad<Stack>().binding {
    val s = State.get<Stack>().bind()
    val (head, tail) = s.first() toT s.drop(1).k()
    State.put(tail).bind()
    yields(head)
}

/**
 * Push written in monad binding style
 */
fun push2(x: Int) = State.monad<Stack>().binding {
    val xs = State.get<Stack>().bind()
    State.put((listOf(x).k() + xs).k()).bind()
    yields(Unit)
}