perf regression with TypeFamilies

Richard Eisenberg rae at
Mon Jun 29 14:14:55 UTC 2020

Hi Harendra,

I saw your comment on a ghc proposal ( 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!


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