State for comprehension sample

arrow-kt-draft.kt

package draft

import arrow.*
import arrow.core.*
import arrow.data.*
import arrow.syntax.option.*
import arrow.typeclasses.*
import arrow.instances.*

typealias Stack = Option<Nel<String>>


fun pop() = State<Stack, Option<String>> { stack ->
    stack.fold({
        None toT None
    }, {
        Nel.fromList(it.tail) toT it.head.some()
    })
}

fun push(s: String) = State<Stack, Unit> { stack ->
    stack.fold({
        Nel.of(s).some() toT Unit
    }, {
        Nel(s, it.all).some() toT Unit
    })
}

fun stackOperations() = State().monad<Stack>().binding {
    val a = push("a").bind()
    val b = pop().bind()
    val c = pop().bind()

    c
}.fix()