It is not clear how Extension Functions are registered. I assume you have to use import (as in Scala), but wiki page is not clear

I hope it is not registered globally as in Groovy and other dynamic languages. 

It's a shame you used a custom mechanism to define extension methods rather than just saying that if the first argument of the static method is the same type as the object, it can be treated as a extension method  or called classicly.

This would mean that all those hundreds of thousands of static methods defined in Java would be usable as extension methods rather than requiring them to switch to using your language. This would mean that you example would actually work even if the 'swap' method was in a plain old Java library.

namespace NS1
{
fun Int.Method() = 10
}
namespace NS2
{
fun Int.Method() = 20
}
namespace NS3
{
import NS1; import NS2;
fun Test() = 10.Method() // ????? NS1.Method or NS2.Method is called ???
}
In C# we can call extensioan methods as static method to solve this problem. And Kotlin??

Hi,

what's about extension properties? for example,

  fun Collection.IsNotEmpty = ! this.IsEmpty

  fun Character.IsVowel = this.toLowerCase() in [ 'a', 'e', 'i', 'o', 'u', 'y' ]

That looks nice, isn't it?

Then, using

  if (mySet?.IsNotEmpty) doSomething(mySet)

looks better than

  if (mySet?!IsEmpty())  doSomething(mySet)

why not?

Maybe it's a typo in the example about generic swap function:

And you mean:

Okay this will be my last attempt to change your minds! Please have a look at Xtend's static extension methods they are doing exactly what I am talking about:

import static extension java.util.Collections.*

This is so much simpler, and I really do feel you are missing a trick not allowing standard Java methods to be treated as extension methods. It seems to me that your arguments are based on the performance of your IDE autocomplete / index size rather than the easy of use plus all of those Java 'extension' methods sitting there waiting for some one to use them.

In fact if Kotlin did the same syntax, maybe one day Java might copy the syntax

One of motivation to introduce extension methods to C# is LINQ. But we can not make analog of LINQ because there is not "expression trees" in kotlin. Do you plan to implement this feature?

There is an important difference between class methods and extension methods regarding inheritance. Maybe you can illustrate is with simple example:

This might be quite unexpected.

I tried to make another example:

So, it looks like there is no way to call the static function foo() - it tries to override virtual function with exactly the same signature, but virtual calls have priority. In my opinion, defining such extension function should lead to a compiler error (with the hint text that such functions can be defined within another function).

I mean, for normal methods we require "override" annotation in such cases.

Also, I could not find resolution rules in the documentation - apparently I learned the trick of re-defining existing functions (within the method) from one of your presentations, not from the wiki.

So, my suggestion would be:

Hi Andrey, Im trying kotlin for the first time, but I can't get your swap() extension example to work from inside my java classes.

Do I need to add something to the calling java class?

This is for the kotlin code, I just pasted your code

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The java code that produces a compiler error, for swap() not being recognized

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