Annotating instances

[Simon Peyton Jones]

Ah, so returning to my original question:

Where is this described/documented?   All I can see here
> <https://github.com/ndmitchell/hlint#readme>is...
>

E.g. is there a HLint user guide?  I'm interested in what the annotations
can and cannot be.  E.g. perhaps ANN could shortcircuit the TH stuff in
some common cases?

Simon



On Mon, 4 Dec 2023 at 12:15, Moritz Angermann <moritz.angermann at gmail.com>
wrote:

> I see. That’s where the confusion comes from. Hlint uses them to allow
> ignoring specific Hlint warnings:
>
> {-# ANN module "HLint: ignore Use string literal" #-}
>
> {- HLINT ignore "Use string literal" -}
>
> and similar. One could maybe argue they should have never been ANN pragmas
> to begin with.
>
> Examples taken from this SO question:
>
> https://stackoverflow.com/questions/19237695/haskell-how-to-tell-hlint-not-to-warning-use-string-literal
>
> On Mon, 4 Dec 2023 at 8:07 PM, Simon Peyton Jones <
> simon.peytonjones at gmail.com> wrote:
>
>> > I don’t think they do anything specific.
>>
>> Now I am truly baffled!  If they don't do anything, why would they be a
>> module at all!  Surely they do something?
>>
>> Simon
>>
>> On Mon, 4 Dec 2023 at 11:58, Moritz Angermann <moritz.angermann at gmail.com>
>> wrote:
>>
>>> 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 at gmail.com> wrote:
>>>
>>>> Luckily Hlint also support HLINT instead which removed the TH pipeline.
>>>>>
>>>>
>>>> Where is this described/documented?   All I can see here
>>>> <https://github.com/ndmitchell/hlint#readme>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 at 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 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
>>>>>>> _______________________________________________
>>>>>>> ghc-devs mailing list
>>>>>>> ghc-devs at haskell.org
>>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>>>>>
>>>>>> _______________________________________________
>>>>>> ghc-devs mailing list
>>>>>> ghc-devs at haskell.org
>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>>>>
>>>>>
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