There should be some way to generate default implementations of the cmp::Eq and cmp::Ord traits (deriving?).

[Dretch]

Now that rust uses the cmp::Eq and cmp::Ord traits for comparisons, there is often lots of error-prone boilerplate required to write impls of these traits. For example look at all the Eq impls in src/libsyntax/ast.rs.

It would good to have some way to auto-generate impls that compare values in the same way that the old polymorphic comparison functions did. This would be similar to the deriving feature of Haskell: http://en.wikibooks.org/wiki/Haskell/Classes_and_types#Deriving.

The syntax might look something like:

struct Bob deriving Eq, Ord {
    some: Boo,
    struct: Foo,
    fields: Hoo,
}

[pcwalton]

+1. I think the syntax should just be impl Bob : Eq;.

[burg]

+1. This would help a lot for Servo. I hope to avoid writing 100+ Eq implementations, one per CSS property :-)

[graydon]

Agree we'll need to do something here; wondering if it can be done without having to hack the compiler a ton. For example: if we finish default methods, and make the default eq method acquire a tydesc and reflect on it and compare that way, then the impl Bob : Eq; declaration can simply mean "just use the default methods for this trait". Yes?

[Dretch]

@graydon That sounds neat.

[pcwalton]

@graydon I don't think it'll be much effort to hack the compiler. This only needs to work for enums and structs; for all the other types the stdlib provides implementations.

[nikomatsakis]

I would really like something more flexible than what Haskell has. In particular, I'd like to be sure it is extensible to user-defined interfaces. For example, it should cover deserializing and the tr interface found in rustc::middle::astencode. That kind of thing is really tedious.

[graydon]

We discussed on IRC today and decided (tentatively) on a scheme for automatically marking an impl as deriving either an all- or any-element structure/wise and enum-wise impl from sub-impls. IRC conversation is over here: http://irclog.gr/#show/irc.mozilla.org/rust/239500

[dckc]

I found myself wishing for this last night.

Does github not have a way to star or +1 an issue? odd.

[catamorphism]

Superseded by the syntax-extension-based implementation of deriving, which works now.