Thanks Richard! I'm happy to see this as I've always felt the flattening story to be unnecessarily complex for type-checker plugins, and in fact uom-plugin has been broken for a long time because of a previous GHC change to flattening skolems.[1,2] I haven't checked but I am moderately hopeful your work will at least make it easier to fix, if not fix it outright.
As part of removing Deriveds, do you plan to change the type-checker plugin interface to drop the redundant argument? Although I suppose GHC could simply pass an empty list to the plugin. Ideally we would have a spec (or at least some tests!) for what constraints get presented to plugins. The current implementation is rather dependent on whatever GHC's solver happens to produce. In my paper [3] I tried to specify plugins based on the published description of OutsideIn(X), but that's far enough from the reality of GHC that it isn't much help in practice. (One point it lacks is any treatment of Deriveds, so I'm happy to see them go!) Cheers, Adam [1] https://github.com/adamgundry/uom-plugin/issues/43 [2] https://gitlab.haskell.org/ghc/ghc/-/issues/15147 [3] https://adam.gundry.co.uk/pub/typechecker-plugins/ On 19/11/2020 04:45, Ben Gamari wrote: > Iavor and Christiaan are two plugin authors that come to mind. > > Ideally the patch would also include some migration documentation in the > release notes. > > Cheers, > > - Ben > > On November 18, 2020 11:20:40 PM EST, Richard Eisenberg > <r...@richarde.dev> wrote: > > Hi all, > > I'm hard at work on two significant refactorings within GHC's constraint > solver. The first is at > https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4149. It removes > flattening meta-variables and flattening skolems. This is a very nice > simplification. Instead, it just reduces type families directly. My other > patch (held up by the first) is at > https://gitlab.haskell.org/ghc/ghc/-/tree/wip/derived-refactor and will > remove Derived constraints, to be replaced by a little bit of cleverness in > suppressing certain confusing error messages. My guess is that either or both > of these will invalidate the current behavior of type-checker plugins. Sadly, > this is not just a case of finding a new function that replicates the old > behavior -- enough is changing under the hood that you might actually have to > rewrite chunks of your code. > > I have never written a type-checker plugin, and so I don't currently have > advice for you. But if you are a plugin author affected by this change and > want help, please reach out -- I would be happy to walk you through the > changes, and then hopefully make a little video explaining the process to > other plugin authors. > > Neither patch will make it for 9.0, but I expect both to be in 9.2. There > may be more where this came from > (https://gitlab.haskell.org/ghc/ghc/-/issues/18965) in the future, but it's > all for a good cause. > > (I have bcc'd plugin authors that I'm aware of. Just adding this in case > you're surprised at receiving this email.) > > Thanks, > Richard -- Adam Gundry, Haskell Consultant Well-Typed LLP, https://www.well-typed.com/ Registered in England & Wales, OC335890 118 Wymering Mansions, Wymering Road, London W9 2NF, England _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
