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

Richard Eisenberg rae at
Thu Nov 19 04:20:40 UTC 2020

Hi all,

I'm hard at work on two significant refactorings within GHC's constraint solver. The first is at 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 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 ( 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.)


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