My main motivation for going with a phantom type over newtypes was that it makes it easier to use in an adhoc fashion without giving up type safety. As a second benefit it seemed a lot easier to implement. Cheers Andreas George Colpitts schrieb am 24.06.2020 um 00:40:

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 mailto: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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