Performance regression on typechecking type families?

Jan Stolarek jan.stolarek at
Sun Nov 30 10:14:34 UTC 2014

I personally have run into exponential compile times with type families. Unfortunately I have not 
had the time yet to reduce my test case to something tractable.


Dnia sobota, 29 listopada 2014, Dr. ERDI Gergo napisał:
> Hi,
> Prompted by this question:
>tations-using-type-families I wrote some code today using closed type
> families and datakinds. Also, as a baseline, I typechecked the code using
> open type families from
> the original question.
> The two files are here:
> On GHC 7.8.3, typechecking took about 45 seconds for each. However, on a
> 'perf' build of GHC 7.9 d8c437b3, with ghc-stage2, the first one took 1m3s
> and the second one 1m12s. A 40% and 60% increase in typechecking time,
> respectively!
> Is this some known regression, something surprising, or is 'perf' simply
> not the right build flavour for this kind of comparison?
> Bye,
>  	Gergo
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