Monad.kt

sealed class Monad<out T> {
    
    object Nothing: Monad<kotlin.Nothing>()
    
    data class Something<out T>(val value: T): Monad<T>()
    
    inline fun<S> flatMap(f: (T) -> Monad<S>): Monad<S> = when(this) {
        is Nothing -> this
        is Something -> f(this.value) // we aren't wrapping f(value) again 
                                      // since it already returns a wrapped value
    }
}