kmonad.kts

sealed class Bag<out THING> {
    object Empty: Bag<Nothing>()
    data class NonEmpty<out THING>(val thing: THING): Bag<THING>()

    // Make a Bag with a THUNG in it
    companion object {
        fun <THUNG> make(thung: THUNG): Bag<THUNG> = NonEmpty(thung)
    }

    // Do stuff to the THING in the Bag by providing a thingToBagOfThang
    // The thingToBagOfThang tells us what to do and returns new Bag which might contain a THANG
    // If our Bag is empty to start with, then it stays empty
    fun <THANG> doStuff(thingToBagOfThang: (THING) -> Bag<THANG>): Bag<THANG> = TODO()
}

// Converts from a string to a Bag
// If the string is empty, we get an empty Bag
// If the string is non-empty, we get a Bag containing an Int of the length of the string
fun toLength(s: String): Bag<Int>  = with(s.length) {
    if (this == 0)
        Bag.Empty
    else
        Bag.make(this)
}

// Basic Expectations
Bag.make("asdf").doStuff(::toLength) == Bag.make(4)
Bag.make("").doStuff(::toLength) == Bag.Empty
Bag.Empty.doStuff(::toLength) == Bag.Empty


// Monad Rules

// Left Identity Law
Bag.make("asdf").doStuff(::toLength) == toLength("asdf")
Bag.make("").doStuff(::toLength) == toLength("")
Bag.make("asdf").doStuff { Bag.Empty } == Bag.Empty

// Right Identity Law
Bag.make("asdf").doStuff(Bag<String>::make) == Bag.make("asdf")
Bag.make("").doStuff(Bag<String>::make) == Bag.make("")
Bag.Empty.doStuff(Bag<String>::make) == Bag.Empty

// Associativity Law
fun reverse(s: String): Bag<String> = if (s.isEmpty()) Bag.Empty else Bag.make(s.reversed())
Bag.make("asdf").doStuff(::reverse).doStuff(::toLength) == Bag.make("asdf").doStuff { reverse(it).doStuff(::toLength) }