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

Hello, I haven't worked on that stuff in Haskell for a long time, but here are some thoughts: - I think plugins should generally be agnostic to the form of the constraints they are given, thus flattening or not should not affect them---after all, the user might have written the constraints in the "flattened" form in the first place. So I think a plugin needs to convert the constraints to whatever form it assumes anyways. - I always thought that derived constraints were a pretty clever way for disseminating information in the constraint solver, is there a note somewhere on what's going to be the new mechanism? -Iavor On Thu, Nov 19, 2020 at 2:21 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

?

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... 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).

On Thu, 19 Nov 2020 at 05:20, Richard Eisenberg 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

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