i think it could be done but it'd probably be slow on anything <java8 and even then, maybe still slow

from what i recall, the way Go handles this does impose quite some performance overhead

it would be interesting to prototype this though. i have a vague spidey-sense feeling that it could be done in a slightly hacky way without altering the kotlin compiler

@mikehearn: I mean lack of type classes. After using rust's traits, it's clear that Kotlin's extension methods are syntactic sugar for freestanding functions

I don't want duck typing. The type system is what attracted me to Kotlin in the first place

yes, but my point was you wanted to be able to run code on "anything that implements the plus function". OK, so let's say there's an interface that expresses that, Addable, but that not all the types you want to use sum() with implement that. under the hood you would need to either specialise the code for any time that's used, or you'd need to do dynamic dispatch at runtime.

it's doable, but not without some performance cost that may be non-obvious

the kotlin stdlib is actually generated from a template system that spits out the specialisations in the kotlin build system

in theory the compiler itself could do that specialisation. but i think generics specialisation is a feature kotlin should leave until Valhalla is close to finalised, as it wouldn't make sense for Kotlin to do something different to Java in this regard. That'd still not allow for arbitrary type classes, but it'd lay the groundwork for doing them C++ template style

Yes. I'm not asking for typeclasses in Kotlin . I'm just saying that my experience is that because it doesn't have them, a little duplicated code is unavailable when you write in a functional style

yes, indeed.

And since most Java APIs are procedural in style the issue is not a big deal in most code we've been writing

We have lots of overload of

use

for example, and often overloads for Iterable<T>, Sequence<T> and T?, but we've learned not to push the functional style too hard

Wrt implementation, isn't Java8 invokedynamic designed for this kind of use case?

yes, you could implement it with invokedynamic, as that allows you to customise the linking

kotlin currently only targets java 6

i am not sure they'd want to introduce a feature into the type system that only has acceptable performance on java 8 though

you could also do it with dynamic proxy interfaces i think. https://www.opencredo.com/2015/07/14/dynamic-proxies-java-part-2/

hmm that article implies performance is better than i'd have expected (at least on java 😎

The performance of proxies varies a lot between VMs.

yeah but that's inevitable when trying to implement such things

KotlinDuckTyping_kt.kt

that works

obviously a real implementation would need to be taught how to handle extension functions.

@JetBrains, are you going to update https://github.com/JetBrains/kotlin-benchmarks ??

java.lang.NoClassDefFoundError: Failed resolution of: Lnl/komponents/kovenant/Promises_apiKt;

Any idea why?

Hello, I would like to know how to create extension like init{} or tableLayout{} for my function. Could anyone give code snippet?

``` class A { ... } fun init(body: A.() -> Unit): A { val a = A(...) a.body() return a }

and if my class is abstract?

Is there a standard Kotlin API for loading resources?