The whole ANN mechanism
https://ghc.gitlab.haskell.org/ghc/doc/users_guide/extending_ghc.html?highli...is,
at root, a good idea. It is pretty generan, and allows annotations to be
arbitrary expressions, provided they are in Typable and Data. And they are
serialised across modules.
In practice though, I'm not sure how widely used they are. I'm not sure
why. I'd love to hear of counter-examples.
Only top level binders can be annotated; but there is no reason in
principle that you should not annotate instance declarations. I don't
think it'd be too hard to implement.
Simon
On Sat, 2 Dec 2023 at 14:51, Jaro Reinders
Hi GHC devs,
I'm working on a GHC plugin which implements a custom instance resolution mechanism:
https://github.com/noughtmare/transitive-constraint-plugin
Currently, I need to place instances in a specific order in a specific file to recognize them and use them in my plugin. I think my life would be a lot easier if I could put annotations on instances. I imagine a syntax like this:
data MyInstanceTypes = Refl | Trans deriving Eq
class f <= g where inj :: f x -> g x
instance {-# ANN instance Refl #-} f <= f where inj = id
instance {-# ANN instance Trans #-} forall f g h. (f <= g, g <= h) => f <= h where inj = inj @g @h . inj @f @g
Using this information I should be able to find the right instances in a more reliable way.
One more thing I was thinking about is to make it possible to remove these instances from the normal resolution algorithm and only allow them to be used by my plugin.
Do you think this would be easy to implement and useful? Or are there other ways to achieve this?
Cheers,
Jaro _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs