Re: PSA: type checker plugins to be affected by upcoming changes to GHC

On Nov 19, 2020, at 5:20 AM, Christiaan Baaij 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.TcP... https://hackage.haskell.org/package/ghc-tcplugins-extra-0.4/docs/src/GHC.TcP... 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 mailto:rae@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