Id Monad

IdMonad.kt

data class IdMonad<out A>(val a: A) {
    companion object {
        fun <A> unit(a: A): Id<A> = Id(a)
    }
    fun <B> flatMap(f: (A) -> Id<B>): Id<B> = f(this.a)
    fun <B> map(f: (A) -> B): Id<B> = unit(f(this.a))
}
...
val x = IdMonad
        .unit("hello")
    .flatMap { Id.unit("$it world") }
println(x.a) // hello world

OptionMonad.kt

sealed class OptionMonad<out A> {
    object None : OptionMonad<Nothing>()
    data class Some<out A>(val value: A) : OptionMonad<A>()

    companion object {
        fun <T> unit(value: A): OptionMonad<A> {
            return Some(value)
        }
    }

    fun <B> flatMap(f: (A) -> OptionMonad<B>): OptionMonad<B> {
        return when (this) {
            is Some -> f(value)
            is None -> None
        }
    }
}

fun findSquareRoot(n: Double): OptionMonad<Double> {
    return if (n >= 0) {
        OptionMonad.unit(Math.sqrt(n))
    } else {
        OptionMonad.None
    }
}

val result = OptionMonad.unit(25.0)
        .flatMap(::findSquareRoot)
        .flatMap(::findSquareRoot)

    when (result) {
        is OptionMonad.Some -> println("Result: ${result.value}")
        is OptionMonad.None -> println("Error: Value is absent")
    }