[Haskell-cafe] Incredibly slow type-level Nub

Roel van Dijk vandijk.roel at gmail.com
Sun Feb 7 15:23:45 UTC 2016

Hi Nickolay,

I'm not sure why your version was *that* slow. But the algorithm was not
optimal. I translated the Haskell Report prelude version of nub to the type
level. That version compiles instantly with the long list. I tested with
GHC 7.10.2.



2016-02-07 14:37 GMT+01:00 Nickolay Kudasov <nickolay.kudasov at gmail.com>:

> 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
> Nub).
> 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,
> Nick
> [1] https://gist.github.com/fizruk/06458fe8b62a1e562af1
> [2]
> http://hackage.haskell.org/package/servant-swagger-1.0.2/docs/Servant-Swagger-Test.html
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