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kotlinlang

I have a `sealed interface Foo { /* ... */ }` and a bunch of existing classes that implement it, and now years later I want to add an `interface Bar : Foo { fun bar() = someDefaultImpl() }`.

from my understanding, I would have to "pollute" the existing code, by doing one of:
1. changing every single existing `Foo`-implementer from `MyClass : Foo` to `MyClass : Foo, Bar`, and also implement `fun bar()` in the definition of `MyClass`
2. modifying `sealed interface Foo` by adding `fun bar()` to `Foo` (and then delete `interface Bar`), and again implement `fun bar()` in the definition of whatever originally implemented `Foo`
instead, is there a way to "auto-implement" `Bar` for everything that implements `Foo`, like <https://users.rust-lang.org/t/what-are-blanket-implementations/49904/2|Rust's blanket implementations>? so that all of this logic live in a different file and can be deleted whenever I want without any code changes as described above?

in other words, I want to emulate Rust traits, in that the logic of the interface can live far away from the implementer of the interface

huh. looks like there are some cursed-looking stuff you can do with extensions here. let's see if I can get this working.

not even cursed. simply using the fact that `Foo` is `sealed` (which I could already guess would came in handy) to write something like, `fun Foo.bar() = when (this) { /* ... */ }` which is even statically resolved, as Kotlin docs explain

what's more, an `else` branch gives you a default impl.

well... first caveat I've encountered is visibility

or are you saying I should flip the inheritance?

Yep, by flipping the inheritance you can use regular methods in the Bar interface, which will be used as a default implementation for all other existing classes

<@UFCJF8B9A> <@UFGCACPE0> coming back to this again, I now see what you mean. I can define the entire impl in the file where I declare `Bar`, and as far as I can see the only change needed outside of this file is to change `sealed interface Foo` to `sealed interface Foo : Bar`. wonderful.