[Haskell-cafe] Incredibly slow type-level Nub

Nickolay Kudasov nickolay.kudasov at gmail.com
Sun Feb 7 20:27:19 UTC 2016

Roel, thanks, your solution works perfectly for me! I should've checked
that implementation myself.

Richard, since Roel's solution works for me, I think I should not file this.

I think type-list [1] is the place on Hackage for list-related type-level
I probably should send a PR adding Roel's Nub there.

Thanks for your help!

[1] https://hackage.haskell.org/package/type-list

On Sun, 7 Feb 2016 at 18:24 Roel van Dijk <vandijk.roel at gmail.com> wrote:

> 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.
> https://gist.github.com/roelvandijk/f115c6b85a3961e1b689
> Regards,
> Roel
> 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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