I read the email thread you refer to but it doesn't seem to explain why you went with solution 2. If you think it worthwhile can you explain here why you chose solution 2?

On Tue, Jun 23, 2020 at 6:55 PM Andreas Klebinger <klebinger.andreas@gmx.at> wrote:
There was a discussion about making UniqFM typed for the keys here a
while ago.
(https://mail.haskell.org/pipermail/ghc-devs/2020-January/018451.html
and following)

I wrote up an MR for one possible approach here:
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3577

It implements solution 2 from that discussion.

Just while getting the patch to typecheck I've already seen a number of
cases where this increased
readability of the code quite a bit so I think it's a good improvement.

If there are strong objections to this solution let me know. In that
case I'm happy to abandon the patch.
If not I will clean it up and get it ready for merging.


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