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

[Bryan Richter]

And in case it proves intractable to provide concise, useful error
messages, perhaps having entries at https://errors.haskell.org (with
examples!) would also be helpful.

On Mon, 15 May 2023 at 18:07, Olaf Klinke <olf at aatal-apotheke.de> wrote:

> On Fri, 2023-05-12 at 17:13 -0400, Brandon Allbery wrote:
> > My first suspicion would be simplified subsumption. Try eta-expanding
> them.
> >
>
> Indeed, the eta-expanded versions do compile with GHC >= 9.
> Now I remember that I've fallen into a similar trap before [1] (and
> Brandon offered the right hint back then, too. Thanks, Brandon!)
> So let me re-cap:
> (1) Haskell is not a category: there are types a, b, c
>     and morphisms f :: a -> b, g :: b -> c
>     such that g.f is not a morphism.
> (2) Haskell is not a lambda calculus [2]: There are terms f such that
>     \x -> f x
>     belongs to the calculus but f does not.
>
> Personally, I would already call the situation improved when the
> compiler instead of Brandon could tell me the fix. From the discussion
> [3]:
>
> > The most interesting thing I've learned from the lengthy threads,
> > along with deficiencies in the testing process for ecosystem
> > breakage, is how much absolute confusion there is over how rank-n-
> > types work, and the limitations and complexities of algorithms
> > pertaining to them. Now that they are in widespread use and on by
> > default in ghc, it seems like an important case where user education
> > is warranted.
>
> I'd love to be educated. And I'd be very pleased if helpful compiler
> error messages were the first entry point for that, so that I can spare
> the patient and helpful Haskell Cafe members some time.
>
> Olaf
>
> [1]
> https://mail.haskell.org/pipermail/haskell-cafe/2023-February/135903.html
> [2]
> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst#costs-and-drawbacks
> [3] https://github.com/ghc-proposals/ghc-proposals/pull/287
>
> that apparently I have not managed to form an in
>
> > On Fri, May 12, 2023 at 5:10 PM Olaf Klinke <olf at aatal-apotheke.de>
> wrote:
> > >
> > > Dear Cafe,
> > >
> > > I'm an infrequent lens user so please forgive me if the below problem
> > > has a trivial solution.
> > >
> > > For a long time the documentation of Control.Lens.Zoom has been
> > > claiming that 'zoom' and 'magnify' can be specialized to the following
> > > types.
> > >
> > > {-# LANGUAGE RankNTypes #-}
> > > -- Type error with GHC 9.0.2 or 9.2.7, okay with GHC 8.8.4
> > > import Control.Lens.Zoom (zoom,magnify)
> > > import Control.Lens.Type (Lens')
> > > import Control.Monad.RWS (RWS)
> > > zoomRWS ::    Monoid w => Lens' s s' -> RWS r w s' a -> RWS r w s a
> > > zoomRWS = zoom
> > > magnifyRWS :: Monoid w => Lens' r r' -> RWS r' w s a -> RWS r w s a
> > > magnifyRWS = magnify
> > >
> > > Indeed GHC 8.8.4 compiles this happily (on lens-4.18.1), but 9.0.2 (on
> > > lens-5.0.1) throws nearly undecipherable errors, claiming that Lens'
> > > can not be matched with some more specialized type involving Magnified
> > > and LensLike.
> > > The source of Control.Lens.Zoom does not differ substantially between
> > > 4.18.1 and 5.0.1 so my guess is it is a type checker issue. Can anyone
> > > explain? On which bug tracker should I raise this issue, if it is
> > > indeed a valid one?
> > > (I pulled the above versions from stackage lts-16.31, -19.5 and -20.20,
> > > respectively and compiled with stack.)
> > >
> > > Thanks
> > > Olaf
> > >
> > > _______________________________________________
> > > Haskell-Cafe mailing list
> > > To (un)subscribe, modify options or view archives go to:
> > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> > > Only members subscribed via the mailman list are allowed to post.
> >
> >
> >
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20230601/212ee8e5/attachment.html>