so I want the kotlin version of declaring an `impl...

# arrow

[sam]

so I want the kotlin version of declaring an implicit Fibble[T]

[simon.vergauwen]

There is no such thing as implicit in Kotlin, this is why all typeclass constraints are regular parameters. Hopefully with KEEP-87 that would change, more on KEEP-87 here https://github.com/Kotlin/KEEP/pull/87.

[sam]

Yep I've been following 87

[simon.vergauwen]

What signature would Fibble have? And what is your use-case?

[sam]

what I'm asking is, what's the arrow teams best advice

[sam]

sealed class Foo {
   object A : Foo()
   object B: Foo()
}

[sam]

interface MyTypeclass<T> { fun doit(t : T) }

[sam]

typeClassInstance.doit(someT)

[simon.vergauwen]

How does the typeclass relate to the ADT?

[sam]

The real domain is I have some errors represented by an ADT say called Error and in a particular case I want to marshall these errors back over HTTP. So I want a typeclass HandleError or whatever I call it, and move the logic for SubscriptionInvalidError or whatever errors I have handled per typeclass.

[simon.vergauwen]

Any reason you don’t want to use when ?

[sam]

not really, just end up with a massive switch

[sam]

when is what I'm doing atm

[simon.vergauwen]

You can split the methods even across files if you want

[simon.vergauwen]

We do this in Arrow as well, and we’ll do that more in the future.

[sam]

I can do when (t) { is SubscriptionInvalidError -> SubscriptionInvalidErrorHandler.doit(t) }

[simon.vergauwen]

There is a couple of tricks you can do

[sam]

ok

[sam]

I won't know the static type of course, I just have a subclass of T. So it wouldn't even work with scala's implicits.

[simon.vergauwen]

Give me a sec 😜

[sam]

😂

[simon.vergauwen]

typeclasses won’t help you there either. There needs to be a boilerplaty when somewhere

[simon.vergauwen]

But I am trying something else not sure if it’s valid Kotlin 😂

[sam]

🙂

[sam]

I suspected the answer might be "stick with when"

[simon.vergauwen]

Nevermind, doesn’t work. You can annotate your ADT with @autofold if you use arrow-meta, move the methods to seperate files.

//DoItA.kt
fun DoItA(a: A)
//DoItB.kt
fun DoItB(b: B)

//ADT.kt
@autofold sealed class Foo {
   fun doIt() = fold(::DoItA, ::DoItB)
   object A : Foo()
   object B: Foo()
}

[simon.vergauwen]

That’ll be about as good as it gets I think

[sam]

fold will do teh when for you ?

[simon.vergauwen]

yep

[sam]

clever

[simon.vergauwen]

it’s inlined 2

[sam]

do you need to use kapt or anything

[simon.vergauwen]

yes for @autofold

[sam]

ok let me read about it

[simon.vergauwen]

It just adds a single fold method as an extension function to your ADT.

[sam]

right

[simon.vergauwen]

inside is just a when.

[sam]

simple

[simon.vergauwen]

Anyway I hope I was of some help.

[sam]

Absolutely

[sam]

You always are

[sam]

Any docs on @autofold btw ?

[simon.vergauwen]

Ouch, I skimmed on those 😄 Feel free to add something if you want, I never gave it enough attention after I wrote it.

[simon.vergauwen]

So much work to do ^^

[sam]

ok

[sam]

Can you link me to a project that uses autofold ?

[sam]

that should be enough for me to set it up

[simon.vergauwen]

https://github.com/JorgeCastilloPrz/ArrowAndroidSamples/blob/master/io/build.gradle

[sam]

👍🏻

[simon.vergauwen]

It's the same setup for all meta programming.

[sam]

ty

[simon.vergauwen]

You're welcome