Can override an extension function somehow? I gues...

# getting-started

[Vampire]

Can override an extension function somehow?
I guess not. 😕

[Joffrey]

Nope, they are technically statically resolved at compile time, so polymorphism doesn't work with extensions

[Joffrey]

That's actually why we don't get iterator-less ArrayList operations from the stdlib, I believe

[Vampire]

:'''-(

[Joffrey]

that said, if you declare twice the same extension function but for different receiver types (say one being a subtype of the other), it would resolve -statically- to the most specific type

[Joffrey]

but it won't work polymorphically, so if you have Child : Parent and declare Child.foo() and Parent.foo(), you will be able to call the "overriden" child function only on variables that are statically known to be of type Child.

Child().foo() // calls the Child extension

val child: Parent = Child()
child.foo() // calls the Parent extension

[Joffrey]

That might be sufficient for you, but since you used the term "override", it might not be.

[Vampire]

It will not.
A library defines an extension function and uses it and I need it to do something different.

[Vampire]

Didn't expect it to work actually

[Joffrey]

A library defines an extension function and uses it and I need it to do something different

Ah. Assuming it's a JVM lib, this compiled down to a static function in the bytecode, so unless you manipulate that lib in some way, you're out of luck

[Joffrey]

I guess it's expected that you can't do that from Kotlin, it goes with the finall-by-default philosophy. This function was not meant to be overridden, so you can't do it. It might be a bit harsh, but it prevents nasty hacks I guess

[Vampire]

Nothing prevents nasty hacks if you are desparate enough.
But I was after some clean way. 😄

[Joffrey]

Nothing prevents nasty hacks if you are desparate enough

True in a way, but ideally the nasty hack will be so hard that it will become more viable to contribute a change to the library code (when possible of course..)

[ephemient]

I wrote a bytecode transform in one of my work projects to work around a bug in a closed-source dependency (replacing a static call with my own function). setting it up is the hard part, the actual code to do so is easy

[Joffrey]

So you were desperate enough 🙂

[Vampire]

I did similar things already, but iirc I used static bytecode weaving with AspectJ or something like that. 😄