Monads: Simple made Easy.

monads.kt

import java.util.Scanner

sealed class Maybe<out T>

data class Just<T>(val value: T) : Maybe<T>()

object None : Maybe<Nothing>() {
    override fun toString() = "None"
}

fun <T, R> bind(m: Maybe<T>, f: (T) -> Maybe<R>) = when (m) {
    is Just<T> -> f(m.value)
    else -> None
}

fun <T> unit(v: T) = Just(v)

fun <T> chain(init: Maybe<T>, vararg effects: (T) -> Maybe<T>) = effects.fold(init, ::bind)

fun readAndAdd(prevNum: Int) = try {
    print("Enter a number: ")
    val num = Scanner(System.`in`).nextLine().toInt()

    Just(prevNum + num)
} catch (e: Exception) {
    None
}

fun main() {
    val result = chain(
        unit(0),
        ::readAndAdd,
        ::readAndAdd,
        ::readAndAdd
    )

    println(result)
}