Have you heard about the state monad? This repo implements this functional programming pattern using kotlin and explains the basics of it.
A stateful computation is a function that takes some state and returns a value along with a new state:
state -> tuple(newState, result)
Example in kotlin, a function that checks if a number is even and keeps track of all evens already checked:
fn isEven(num: Int, totalOfEvens: Int): Pair<Boolean, Int> =
if (num % 2 == 0) Pair(true, totalOfEvens.inc) else Pair(false, totalOfEvens) What if we abstract the previous function in something more generic?
Let's wrap our initial function in a new type:
state -> (state, result) to State(f: state -> (state, result))In kotlin:
data class State<STATE, out RESULT>(val run: (STATE) -> Pair<RESULT, STATE>)And let's refactor our previous code:
fun isEven(num: Int): State<Boolean, Int> = State { state ->
if (num % 2 == 0) Pair(true, state.inc) else Pair(false, state)
}Simple, state monad is just a datatype wrapping a stateful computation
We can not finish without mentioning that state is a monad, therefore it should implement map and flatmap functions.
In this example it would be missing to implement get and put functions that usually come alongside the pattern as well.