perf regression with TypeFamilies

[Richard Eisenberg]

Hi Harendra,

I saw your comment on a ghc proposal (https://github.com/ghc-proposals/ghc-proposals/pull/343#issuecomment-650797297) that said you experienced perf regressions with -XTypeFamilies enabled (but no other changes). Are these reproducible in a way that can be shared publicly? And are the regressions in compile times or run times?

-XTypeFamilies enables -XMonoLocalBinds, which disables let-generalization on some nested lets. It is thus just barely conceivable that different Core is produced depending on this extension, and that there may be a possibility of performance changes. But this would be unexpected, and something worth investigating.

In other words: if you can, do please post a bug -- simply enabling -XTypeFamilies should not slow anything down!

Thanks,
Richard