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

Richard Eisenberg rae at
Thu Nov 19 19:04:36 UTC 2020

> On Nov 19, 2020, at 5:20 AM, Christiaan Baaij <christiaan.baaij at> 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): <>
> 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. :)


> On Thu, 19 Nov 2020 at 05:20, Richard Eisenberg <rae at <mailto: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.)
> Thanks,
> Richard

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