Functional Kotlin

Corecursion with State

State is beneficial on corecursion; we can rewrite our old examples with State:

fun <T, S> unfold(s: S, f: (S) -> Pair<T, S>?): Sequence<T> {   val result = f(s)   return if (result != null) {      sequenceOf(result.first) + unfold(result.second, f)   } else {      sequenceOf()   }}

Our original unfold function use a function, f: (S) -> Pair<T,S>? which is very similar to State<S, T>:

fun <T, S> unfold(s: S, state: State<S, Option<T>>): Sequence<T> {    val (actualState: S, value: Option<T>) = state.run(s)    return value.fold(            { sequenceOf() },            { t ->                sequenceOf(t) + unfold(actualState, state)            })}

Instead of having a lambda (S) -> Pair<T, S>?, we use State<S, Option<T>> and we use the function fold from Option, with an empty Sequence for ...