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

[Olaf Klinke]

> 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