[Haskell-cafe] Incredibly slow type-level Nub

Nickolay Kudasov nickolay.kudasov at gmail.com
Sun Feb 7 13:37:17 UTC 2016

Dear Cafe,

I have faced a type-level performance problem (with GHC 7.10.3).
I am trying to implement type-level Nub which removes duplicates from a
type-level list.

My implementation [1] with an example of a list of length 20 takes forever
to compile. If you reduce the size of the example list (e.g. to 15 items),
it will compile faster. Also, if you remove the Nub application from
exampleVals, it compiles instantly (this is how I know the problem is in

My question is how can I speed up type-level Nub?
And less importantly why exactly is it this slow?

The practical application for this is building automatic tests [2] for
servant-swagger. There I am interested in generating exactly one test for
each distinct type.

Kind regards,

[1] https://gist.github.com/fizruk/06458fe8b62a1e562af1
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20160207/32989c66/attachment.html>

More information about the Haskell-Cafe mailing list