Free monad, but in Kotlin!

freesample.kt

fun main() {
    
    val result = Free.fx<ForStore, String> { 
        put("arrow", "huita").bind()
        update<String>("arrow") { "let's me do this" }.bind()
        val arrow = get<String>("arrow").bind()
        put("arrow-huita", "I have spoken").bind()
        delete("arrow-huita")
        arrow ?: ""
    }
    
    println(result)
    
}

fun <F, A> Free.Companion.fx(comp: suspend MonadSyntax<FreePartialOf<F>>.() -> A): Free<F, A> =
    Free.monad<F>().fx.monad(comp).fix()

class ForStore private constructor()

sealed class StoreA<A> : Kind<ForStore, A>

data class Put<A>(val key: String, val value: A) : StoreA<Unit>()
data class Get<A>(val key: String) : StoreA<A?>()
data class Delete(val key: String) : StoreA<Unit>()

typealias Store<A> = Free<ForStore, A>

fun <T> put(key: String, value: T): Store<Unit> = Free.liftF(Put(key, value))

fun <T> get(key: String): Store<T?> = Free.liftF(Get(key))

fun delete(key: String): Store<Unit> = Free.liftF(Delete(key))

fun <T> update(key: String, upd: (T) -> T): Store<Unit> {
    return Free.fx<ForStore, Unit> {
        val value = get<T>(key).bind()
        val res = value?.let(upd)?.let { put(key, it) } ?: Free.just(Unit)
        res.bind()
    }.fix()
}