> On Nov 19, 2020, at 5:20 AM, Christiaan Baaij <christiaan.ba...@gmail.com> > wrote: > > I always forget what flattening did/does. Is it the thing where it turns a > "complex" type family application: > > F (G y) (H z) ~ a > > into: > > G y ~ p > H z ~ q > F p q ~ a > > ?
Yes, but it's worse. It would actually leave us with G y ~ p H z ~ q F p q ~ r r ~ a > > If so, then I'm all for removing that. Since I actually wrote/hacked a > function that "reverts" that process (for [G]ivens only): > https://hackage.haskell.org/package/ghc-tcplugins-extra-0.4/docs/src/GHC.TcPluginM.Extra.html#flattenGivens > > <https://hackage.haskell.org/package/ghc-tcplugins-extra-0.4/docs/src/GHC.TcPluginM.Extra.html#flattenGivens> > Which I use in all of my plugins. (PS it should perhaps be called > "unflattenGiven"? like I said, I always get confused about flatten vs > unflatten). Yes, this would be unflattening. Glad to know this would work well with your plugins. :) Richard > > On Thu, 19 Nov 2020 at 05:20, Richard Eisenberg <r...@richarde.dev > <mailto: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 > <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 > <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 > <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
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