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

Christiaan Baaij christiaan.baaij at
Thu Nov 19 10:20:37 UTC 2020

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


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):
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

On Thu, 19 Nov 2020 at 05:20, 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.)
> Thanks,
> Richard
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