Feature request: support derive for traits

[gmlewis]

Context: https://github.com/gmlewis/moonbit-ray-tracer

In this repo, I was forced to write:

pub(open) trait Object {
  ...
  output(Self, Logger) -> Unit
  to_string(Self) -> String
}

instead of (preferred):

pub(open) trait Object {
  ...
} derive(Show)

in order to support inspect! in black-box unit tests, but the compiler doesn't allow it.
(Also, I wanted to add op_equal to support assert_eq! but couldn't get it to work with trait objects... but that's probably a different issue that is unrelated to this one.)

It might be nice to be able to derive any methods for traits, like Show, Eq, FromJson, ToJson, etc.

[gmlewis]

As an aside... now I'm remembering that I got a weird error message about Self showing up twice when I tried to add op_equal(Self, Self) -> Bool to the trait... very strange.

[peter-jerry-ye]

pub(open) trait Object {
...
} derive(Show)

Does

pub(open) trait Object : Show {
 // everything other than to_string / output
}

Work for you?

As an aside... now I'm remembering that I got a weird error message about Self showing up twice when I tried to add op_equal(Self, Self) -> Bool to the trait... very strange.

Maybe you are using it as trait object?

[gmlewis]

Sure, sounds good. So multiple derives would be trait Object : Show, Eq, ToJson, FromJson {...} for example ?

Maybe you are using it as trait object?

I didn't think so, but will have to try again to test it out.

[gmlewis]

Again, feel free to close if this is already the case.

[peter-jerry-ye]

Sure, sounds good. So multiple derives would be trait Object : Show, Eq, ToJson, FromJson {...} for example ?

It's trait Object : Show + Eq + ToJson + FromJson.

As an aside... now I'm remembering that I got a weird error message about Self showing up twice when I tried to add op_equal(Self, Self) -> Bool to the trait... very strange.

This could be a bug if you are using a typealias with a trait. Should be fixed soon.

[peter-jerry-ye]

Transferring to moonbit-docs since it's a missing topic in document.