Annotating instances

[Moritz Angermann]

Any ANN annotation triggers the TH pipeline and makes them really painful
to work with, in non-stage2 settings. Lots of Hlint annotations use ANN and
then you have iserv be triggered for each module that has an ANN annotation.

Luckily Hlint also support HLINT instead which removed the TH pipeline.

That alone is enough for me personally to recommend against using ANN if
there is an alternator option to anyone who asks me.

On Mon, 4 Dec 2023 at 5:01 PM, Simon Peyton Jones <
simon.peytonjones at gmail.com> wrote:

> The whole ANN mechanism
> <https://ghc.gitlab.haskell.org/ghc/doc/users_guide/extending_ghc.html?highlight=ann#source-annotations>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 <jaro.reinders at gmail.com>
> wrote:
>
>> 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
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>>
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