Where has Functor disappeared from Arrow? In 0.12 it was there: <https://arrow-kt.io/docs/0.12/arrow...

# arrow

[Ville Peurala]

Where has Functor disappeared from Arrow? In 0.12 it was there: https://arrow-kt.io/docs/0.12/arrow/typeclasses/functor/

How do I implement a Functor for my own type in 1.1.3?

[simon.vergauwen]

Hey @Ville Peurala,

It was deprecated in 0.12.x, and removed in 0.13.x. Functor (& co) was a abstraction that didn't work well in Kotlin, we tried to look for many solutions and alternatives and we ended removing it in favor of the monadic suspend DSL system we have now.

Not sure if you've been able to take a deep look at it? We introduced the first version in 0.13.x, refining it in 1.x.x and I think we've nailed it for Arrow 2.x.x which we're actively working on now.

[Ville Peurala]

Thanks for the info @simon.vergauwen. Can you give me a link to some up-to-date documentation about the monadic suspend DSL system?

[simon.vergauwen]

Here is the PR with the design document for Arrow 2.x.x, https://github.com/arrow-kt/arrow/pull/2797

And in similar fashion how we design unwrapped monadic DSLs for a different monad Resource, also known as Managed. https://github.com/arrow-kt/arrow/pull/2786

The current 1.x.x documentation can be found here, https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core.continuations/-effect/

[simon.vergauwen]

Not sure if that solves your current problem, but I'd be happy to take a closer look at your use-case if you could provide some more details or code 🙂

[Ville Peurala]

Thanks a lot! I can provide some details.

I have an interface `Timed<A>`:

```interface Timed<A> {

fun result(): A

fun startMs(): Long

fun endMs(): Long

}```

Now I would like to make Timed a Functor so that users could always map overt the result without affecting the timings.

[Ville Peurala]

I could just add a function fun <B> map(fn: (A) -> B): Timed<B> to my interface but I have the feeling that it is not an optimal solution.

[simon.vergauwen]

This is mostly oriented around monadic DSLs so that implies bind or flatMap. Since in Kotlin we don't have a need for Functor for collections.

I.e. you can do list.map { either.bind() } safely within a DSL to replace traverse 🎉

So in this case I would opt for adding map, or I think you would need to define zip or Applicative as well and then you can also implement Monad.

[simon.vergauwen]

The problem for zip is how do you combine startMs and endMs does it take the earliest startMs and the latest endMs ? 🤔

[simon.vergauwen]

If these are questions that don't make sense for your use-case I would opt for simply adding the map function.

[simon.vergauwen]

Tip: if you implement it as inline you will be able to call suspend functions from the lambda, but you'll need to write map as an extension function on Timed<A> since inline functions need to be final 😉

[Ville Peurala]

Thanks for the ideas. Yeah, the definition of an Applicative for Timed<A> would need some thinking about, but I'm not sure yet whether I need it. I might need a Monad too, but I'll start with a Functor now and see where that takes me.

[simon.vergauwen]

I quickly whipped something up quickly.

[simon.vergauwen]

zip is now timed { transform(timedA.bind(), timedB.bind(), timedC.bind(), ...) } and the strategy it applies over combining startMs and endMs is parameterised.

[simon.vergauwen]

https://gist.github.com/nomisRev/e1b5cf39ca4f2931365a1a30f9e0c565

[Ville Peurala]

Wow! You wrote my code for me! Thanks. 😄 👍