A follow up to my earlier question on Rx with Arro...

# arrow

[Mark Fisher]

A follow up to my earlier question on Rx with Arrow. I'm trying to write a comprehension that combines calls to functions that return Rx Single objects. For example:

class ArrowRxTests {
    @Test
    fun `comprehension with arrow and rx`() {
        val x = SingleK.fx {
            val (count) = getCountSingle       // this is an async call using a SingleK.monadDefer
            val newCount = increment(count)    // this is an async call returning Single<Int>.... how do i use it here?
            newCount
        }.value()

        assertThat(x.blockingGet()).isEqualTo(2)
    }

    private fun getCount() = 1
    private fun <F> getCountAsync(MS: MonadDefer<F>) = MS.later { getCount() }
    private val getCountSingle: SingleKOf<Int> = getCountAsync(SingleK.monadDefer())

    private fun increment(i: Int): Single<Int> = Single.just(i + 1)
}

This works if increment just returns an int, but how do I use increment() in the comprehension when it already returns a Single?
I want to somehow keep the flow of the code inside SingleK.fx {} but using functions that return Rx types

[simon.vergauwen]

What you’re doing in the first line is “extracting” the value from getCountSingle using operator fun component1() or destructuring.

[simon.vergauwen]

This is an alias for the functions operator fun not() or fun bind().

So val (count) = getCountSingle is the same as val count = getCountSingle.bind() or val count = !getCountSingle.

[simon.vergauwen]

So you can do the same thing for increment .

[simon.vergauwen]

val x = SingleK.fx {
  val count = !getCountSingle
  val newCount = !increment(count)
  newCount
}.value()

[Mark Fisher]

how do I import the "!"? it's not coming up for me

[simon.vergauwen]

It should be available within fx

[simon.vergauwen]

It’s enabled through the receiver AsyncSyntax<ForSingle> which should be the scope of this within fx.

[Mark Fisher]

message has been deleted

[Jannis]

Is there some sort of function from Single -> SingleK like k() ? because that is what you need first^^

[simon.vergauwen]

Oh my bad. Those are only available for SingleK.

[simon.vergauwen]

We need to add binds for concrete types in Syntax classes for wrappers such as arrow-fx-rx and arrow-fx-reactor

[simon.vergauwen]

I’m afraid this requires changes in Arrow, but I’d be happy to help you implement them if you’re interested in contributing @Mark Fisher

[Mark Fisher]

!increment(count).k() worked

[simon.vergauwen]

Yes! That works because the operations are defined for Kind<ForSingleK, A> and not for Single<A> directly.

[simon.vergauwen]

All of this boilerplate will be gone with arrow-meta, and these confusing situations cannot occur anymore.

[Mark Fisher]

how far is meta off btw?

[Mark Fisher]

and many thanks for the help on this!

[simon.vergauwen]

I think a preview will become available soon. I am not 100% sure, but #arrow-meta will be able to give you a better answer. Everyone there is working hard and it’s progressing too fast for me to be able to keep track 😅