"Direct" implementation of traits about dora HOT 6 CLOSED

I think the hardest part is deciding how the class syntax should look like.

One problem is that if the class already has an inheritance relationship, the syntax would look kinda weird:

class Foo(...) : Bar(...) <keyword> Comparable {
  fun compareTo(rhs: Foo) -> Int = ...
}

It feels like this creates a much stronger visual connection between Bar and Comparable than between Foo and Comparable, which I believe is undesirable.

One approach would be to use only operators like this (and line : up with it's use in generics ...

class Foo(...) < Bar(...) : Comparable {
  fun compareTo(rhs: Foo) -> Int = ...
}

... but I feel this could create a lot of line-noise and would make reading harder.

On the other hand, I kind of dislike the use of : for different things, so maybe going the other way around and giving both inheritance and traits a keyword is a good way to tackle this issue:

class Foo(...) extends Bar(...) supports Comparable {
  fun compareTo(rhs: Foo) -> Int = ...
}

Based on the exact keyword chosen for direct trait implementations (I'd consider with or supports as workable options, but I don't have a too strong opinion on that), the naming scheme of traits would need to be adapted:

... supports Comparable
... supports Sortable
... supports Equals
... supports Identity
... supports Hash
... supports Default

... with Comparable
... with Sortable
... with Equals
... with Identity
... with Hash
... with Default

In both cases there are combinations that sound off.

Maybe there is a consistent selection of names, but I have a hard time figuring it out:

supports: Comparing, Sorting, Equality, Identity, Hashing, Defaulting(?)
with:     Comparable, Sortable, Equatable, Identifiable, Hashable, Defaultable

Alternatively, we could use impl (which raises the question why extends is not abbreviated, but impl is:

class Foo(...) extends Bar(...) impl Comparable {
  fun compareTo(rhs: Foo) -> Int = ...
}

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I have worked on this and added the required logic and a struct Supports to the parser and ast; the Classstruct received a new field next toparent_class`.

I think the next step would be to add the direct implementations of traits to the VM's list of impls.

@dinfuehr Do you have a suggestion at which point this should happen?

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AFAIU this feature should just be syntactic sugar, so ideally it should be implemented exclusively in the front-end and generate the same data in vm.impls, but not sure whether ImplData is ready for this now. So this might have to be changed first.

When you want to work on impl, IMHO it would be better to work on allowing to implement traits for generic classes (so implementing Default for Option[T] doesn't work).

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so implementing Default for Option[T] doesn't work

Do you mean "does work", or am I misunderstanding something?

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Oh sorry, I meant that implementing Default for Option[T] doesn't work at the moment.

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Ahh, ok, thanks, understood!

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