[Haskell-cafe] Type-level numbers problem
k-bx at k-bx.com
Mon Aug 18 19:55:43 UTC 2014
On Mon, Aug 18, 2014 at 10:51 PM, Dmitry Dzhus <dima at dzhus.org> wrote:
> 18.08.2014, 23:16, "Konstantine Rybnikov" <k-bx at k-bx.com>:
> > The one that's below "main" is working properly, while the one on top of
> it gives this error:
> > typenats.hs:17:16:
> > No instance for (Op D (0 + 1)) arising from a use of `f'
> > Possible fix: add an instance declaration for (Op D (0 + 1))
> > In the expression: f (VJ i :: VJ D (0 + 1))
> > In an equation for `f': f (VJ i) = f (VJ i :: VJ D (0 + 1))
> > In the instance declaration for `Op D 0'
> Hello Konstantine,
> I think you're using GHC 7.6.x series which had no solver
> for type-level natural numbers, resulting in 0+1
> not typechecking with 1.
> You may find some history in https://ghc.haskell.org/trac/ghc/ticket/4385.
> With GHC 7.8 this compiles fine.
> Probably you can overcome the issue by providing
> the required instances of `+` type family
> by hand (up to the desired number), but it's hackish
> and may be tedious without TH. Just switch to 7.8 already.
Dmitry, you are 100% correct. Thank you very much! Moving to 7.8 straight
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