Deriving mechanism

[gefjon]

I'm interested in this, and it seems like it would be a fun way to dip my toes into the compiler (until now I've focused on the standard library). Before I start working, tho, I'd like to know what syntax we'd prefer.

Prior art are Haskell:

data Foo = Bar | Baz deriving (Eq)

and Rust:

#[derive(PartialEq)]
enum Foo {
  Bar,
  Baz,
}

Neither of these has obvious Coalton analogues. My first intuition is to have a separate top-level form derive-instance which looks like:

(define-type Foo Bar Baz)
(derive-instance (Eq Baz))

[stylewarning]

I like the Coalton example syntax a lot.

[macrologist]

+1 for this