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

Ben Gamari ben at
Thu Nov 19 04:45:59 UTC 2020

Iavor and Christiaan are two plugin authors that come to mind.

Ideally the patch would also include some migration documentation in the release notes.


- Ben 

On November 18, 2020 11:20:40 PM EST, Richard Eisenberg <rae at> wrote:
>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.)
>ghc-devs mailing list
>ghc-devs at

Sent from my Android device with K-9 Mail. Please excuse my brevity.
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