I don’t think they do anything specific. They just function as a marker to Hlint to find when parsing the source files. Here is one of the original issues we had: 
https://github.com/ndmitchell/hlint/issues/1251

Simply by not being ANN, it doesn’t trigger the Templar Haskell machinery and thus does not cause compilation slowdowns or iserv needs (e.g. render the module impossible to cross compiler for stage1 cross compilers with not TH support).

On Mon, 4 Dec 2023 at 7:45 PM, Simon Peyton Jones <simon.peytonjones@gmail.com> wrote:
Luckily Hlint also support HLINT instead which removed the TH pipeline.

Where is this described/documented?   All I can see here is

For {-# HLINT #-} pragmas GHC may give a warning about an unrecognised pragma, which can be suppressed with -Wno-unrecognised-pragmas.

which mentions HLINT pragmas but says nothing about what they do.

Simon

On Mon, 4 Dec 2023 at 09:05, Moritz Angermann <moritz.angermann@gmail.com> wrote:
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@gmail.com> wrote:
The whole ANN mechanism 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@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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