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

[Bryan Richter]

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.)



On Fri, 2 Jun 2023 at 03:33, Viktor Dukhovni <ietf-dane at dukhovni.org> wrote:

> On Thu, Jun 01, 2023 at 11:58:47PM +0200, Olaf Klinke wrote:
>
> > The thing is, the error message is rather general, which was my
> > original complaint. I haven't got a GHC-9.6.1 installation, so what is
> > the error code when compiling the following minimal example?
>
> Essentially the same error text with GHC 9.2.7:
>
>     foo.hs:13:10: error:
>         • Couldn't match type ‘()’ with ‘forall t. t’
>           Expected: Void -> ()
>             Actual: () -> ()
>         • In the expression: id :: () -> ()
>           In an equation for ‘final’: final = (id :: () -> ())
>        |
>     13 | final = (id :: () -> ())
>        |          ^^^^^^^^^^^^^^
>
> and GHC 9.6.2:
>
>     foo.hs:13:10: error: [GHC-83865]
>         • Couldn't match type ‘()’ with ‘forall t. t’
>           Expected: Void -> ()
>             Actual: () -> ()
>         • In the expression: id :: () -> ()
>           In an equation for ‘final’: final = (id :: () -> ())
>        |
>     13 | final = (id :: () -> ())
>        |          ^^^^^^^^^^^^^^
>
> --
>     Viktor.
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