Implementation of Type level warnings

[Richard Eisenberg]

I've responded on the MR. Thanks for stepping up to do this!

Richard

> On Dec 30, 2019, at 5:53 AM, Rowan Goemans <goemansrowan at gmail.com> wrote:
> 
> Hello everyone,
> 
> A couple of days ago I tried to implement type level warnings [1]. I have got something
> now that sort of works but issues the same warning multiple times and not on the correct
> location. It's currently a WIP merge request [2]. I would like some help/pointers
> to continue the implementation
> 
> The approach I have taken is to issue the warning once TypeWarning is rewritten.
> However, this has severe problems since now a warning is issued multiple times and
> at different locations due to the rewriting happening multiple times. I don't know
> what the best location is to do this check and issue the warning. Prior to
> constraint solving isn't a good place since the TypeWarning could be nested
> deeply in some constraint. Afterwards also isn't doable since then the
> TypeWarning constraint is gone.
> 
> An alternative could be to not implement any instance in for TypeWarning
> similar to TypeError and special case it somehow.
> 
> Any help is appreciated!
> 
> [1] https://gitlab.haskell.org/ghc/ghc/issues/17027 <https://gitlab.haskell.org/ghc/ghc/issues/17027>
> [2] https://gitlab.haskell.org/ghc/ghc/merge_requests/2361 <https://gitlab.haskell.org/ghc/ghc/merge_requests/2361>
> 
> Best regards,
> 
> Rowan Goemans
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