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kevin.cianfarini

12/04/2020, 7:26 PM
Are polymorphic calls in Kotlin/Native static or dynamic?
👀 1
d

Dominaezzz

12/04/2020, 8:32 PM
Dynamic (Mostly). Monomorphisation is not performed and there's no JIT.
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kevin.cianfarini

12/04/2020, 8:34 PM
Gotcha. If monomorphisation isn't performed, how does K/N deal with generics? For method calls, why choose to perform polymorphism dynamically instead of statically?
Do you have any resources I can read to get a grip on how K/N works under the hood?
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Dominaezzz

12/04/2020, 8:36 PM
Ha! I wish such resource existed.
Generics are erased like on the JVM.
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kevin.cianfarini

12/04/2020, 8:36 PM
Ouch. Do you know why this design decision was made?
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Dominaezzz

12/04/2020, 8:37 PM
Compatibility with Kotlin/JVM
If they were to break compat, might as well build another Rust lol
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kevin.cianfarini

12/04/2020, 8:37 PM
Was it mainly just to stay aligned with the JVM? I know 
reified
is a thing...but that could be a no-op in native
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Dominaezzz

12/04/2020, 8:38 PM
Yeah multiplatform would be incredibly difficult if they didn't align.
refied is a tricky one
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kevin.cianfarini

12/04/2020, 8:38 PM
There isn't direct compat between K/N and JVM, it's just implementation details of how language features are compiled, right?
Yeah multiplatform would be incredibly difficult if they didn't align.
Do you have any examples that highlight this?
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Dominaezzz

12/04/2020, 8:38 PM
Reified functions are inlined at compile time, you could almost call it monomorphisation.
😆 1
Don't have any examples of that sadly.
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kevin.cianfarini

12/04/2020, 8:40 PM
Womp. Thanks though!
d

Dominaezzz

12/04/2020, 8:40 PM
Even the whole concurrency rules that are enforced at runtime already make multiplatform somewhat difficult.
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kevin.cianfarini

12/04/2020, 8:40 PM
Yeah, and I understand that's being worked on. I'm mainly asking this out of curiosity. I'm going to keep digging and see if I can get some answers
d

Dominaezzz

12/04/2020, 8:41 PM
Polymorphism also can't be performed statically btw.
JVM can pull it off because it literally compiles the code at runtime.
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kevin.cianfarini

12/04/2020, 8:41 PM
Give me a second, because I have a precise example in my head and I want to know where it breaks down
👍🏼 1
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Dominaezzz

12/04/2020, 8:41 PM
Yeah best option now is to read source code and perhaps watch KotlinConf videos.
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kevin.cianfarini

12/04/2020, 8:44 PM
So I have an interface, 
A
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interface A { fun foo() }
and it has two implementations, 
B
and 
C
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class B : A { override fun foo() = Unit }
class C : A { override fun foo() = println("") }
When making a call to an instance of B or C, the JVM will look into the Virtual method table to figure out which function to be invoked. I'm not quite sure why this is necessary, as this seems like it could be totally enforced by the compiler? I have very limited compiler knowledge, so please bare with me
Above, "the compiler" refers to konan
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Dominaezzz

12/04/2020, 8:45 PM
Ohh
You've kinda got a point there.
Konan will "perform it statically" in that case because polymorphism is not being used.
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kevin.cianfarini

12/04/2020, 8:46 PM
What if I said this
d

Dominaezzz

12/04/2020, 8:47 PM
On the JVM all functions are overridable by default so it must check the vtable.
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kevin.cianfarini

12/04/2020, 8:47 PM
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val bar = listOf(C(), B())
bar.forEach { it.foo() }
that's certainly polymorphic
d

Dominaezzz

12/04/2020, 8:48 PM
Yes definitely.
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kevin.cianfarini

12/04/2020, 8:48 PM
would that be able to be resolved by the compiler alone?
d

Dominaezzz

12/04/2020, 8:48 PM
Hmm, tricky. A sufficiently advance compiler yes, but konan is not that haha.
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kevin.cianfarini

12/04/2020, 8:48 PM
Certainly not 😂
tis but a 👶
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Dominaezzz

12/04/2020, 8:49 PM
Indeed
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kevin.cianfarini

12/04/2020, 8:49 PM
Thanks for entertaining this lol. Looks like I've got some digging to do
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Dominaezzz

12/04/2020, 8:50 PM
Good talk. Have fun indulging in the compiler internals.
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kevin.cianfarini

12/04/2020, 9:10 PM
After looking around the K/N github, it seems like they answer questions tagged with 
kotlin-native
on stackoverflow
After doing a bit of research on static dispatch (and rust) I've come to some partial conclusions. Consider the following function
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interface A
class B : A
class C : A

fun foo(a: A)
If the same method that rust employs were to be taken in Kotlin, then polymorphism is really shorthand for generic bounds. It could be transformed into the following
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fun <T : A> foo(a: T)
Using monomorphism, the compiler would search through all usages of this function to see what concrete types actually call this function. And then it would implement them for those classes.
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fun foo_b(a: B)
fun too_c(a: C)
Could this method be employed for class inheritance and not interfaces? I don't know. Rust doesn't allow inheritance and this might be the reason they can pull off total static dispatch. At the very least, it seems like Kotlin might be able to do it for interfaces.
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Dominaezzz

12/05/2020, 8:37 PM
Yes, I think it's being considered but for the far future.
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