[Haskell-cafe] Type-level numbers problem

Dmitry Dzhus dima at dzhus.org
Mon Aug 18 19:51:29 UTC 2014

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.

More information about the Haskell-Cafe mailing list