Monads
A monad is a functor type that defines a flatMap
(or bind
, in other languages) function, that receives a lambda that returns the same type. Let me explain it with an example. Luckily for us, List<T>
defines a flatMap
function:
fun main(args: Array<String>) { val result = listOf(1, 2, 3) .flatMap { i -> listOf(i * 2, i + 3) } .joinToString() println(result) //2, 4, 4, 5, 6, 6 }
In a map
function, we just transform the List
value's content, but in flatMap
, we can return a new List
type with less or more items, making it a lot more potent than map
.
So, a generic monad will look like this (just remember that we don't have higher-kinded types):
interface Monad<C<_>>: Functor<C> { //Invalid Kotlin code fun <A, B> flatMap(ca:C<A>, fm:(A) -> C<B>): C<B> }
Now, we can write a flatMap
function for our Option
type:
fun <T, R> Option<T>.flatMap(fm: (T) -> Option...