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

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-si... [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 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.