[Haskell-cafe] type error when specializing lens zooms in ghc >= 9.0

[Bryan Richter]

Hi Olaf,

I don't know the precise background to the current system for assignment of
error codes, but I do know that there is ongoing work to provide more
structure to errors. These instances of overly-general codes (and
hypothetically some that may be overly-specific) are artifacts of a rather
young process.

I added some of your suggestions to the ticket.


On Fri, 2 Jun 2023 at 17:16, Olaf Klinke <olf at aatal-apotheke.de> wrote:

> > Hm, yikes, I tend to agree this is a very general error message. Here
> > are
> > all the places "83865" shows up in the code base:
> >
> >
> >
> https://gitlab.haskell.org/search?search=83865&project_id=1&group_id=2&search_code=true&repository_ref=master
> >
> > I opened https://gitlab.haskell.org/ghc/ghc/-/issues/23466 to follow
> > up.
> > Feel free to weigh in. (Contact me if you need an account - spam
> > measures
> > currently include manual approvals for new accounts.)
> >
>
> Thanks, Bryan.
>
> This made me wonder what the current system behind assignment of error
> codes is. I'd expect five-digit error codes to be *more* specific than
> the general shape of error message emitted by ghc, but here it is
> evidently less specific. 82865 stands for the error message constructor
> TypeEqMismatch [1] which is a record [2] with many fields. In
> particular, the teq_mismatch_expected and teq_mismatch_actual fields
> hold structural information: a ForAllTy [3] may hint at simplified
> subsumption.
>
> Thus my proposal would be that error codes are composite values, with
> one part (as currently implemented) in bijection to the error message
> constructor and another part determined by a function of the actual
> error value. This function has to be chosen to provide sufficient
> abstraction from the actual situation but e.g. differentiate between
> constructors of the Type type. For example, why don't the cases
> differentiated by the pretty-printer [4] not own their own error codes?
>
> Olaf
>
> [1]
> https://gitlab.haskell.org/ghc/ghc/-/blob/master/compiler/GHC/Types/Error/Codes.hs
> [2]
> https://hackage.haskell.org/package/ghc-9.6.1/docs/src/GHC.Tc.Errors.Types.html#TypeEqMismatch
> [3]
> https://hackage.haskell.org/package/ghc-9.6.1/docs/src/GHC.Core.TyCo.Rep.html#ForAllTy
> [4]
> https://hackage.haskell.org/package/ghc-9.6.1/docs/src/GHC.Tc.Errors.Ppr.html#pprMismatchMsg
>
>
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