On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly <arnaud.oq...@gmail.com> wrote:
> Hello,
> I am trying to understand and use the Nat n type defined in the
> aforementioned article. Unfortunately, the given code does not compile
> properly:
[snip]
> instance (Nat n) => Nat (Succ n) where
> toInt _ = 1 + toInt (undefined :: n)
[snip]
> And here is the error:
>
> Naturals.hs:16:18:
> Ambiguous type variable `n' in the constraint:
> `Nat n' arising from a use of `toInt' at Naturals.hs:16:18-39
> Probable fix: add a type signature that fixes these type variable(s)
You need to turn on the ScopedTypeVariables extension (using {-#
LANGUAGE ScopedTypeVariables #-} at the top of your file, or
-XScopedTypeVariables at the command line). Otherwise, the 'n' in the
class declaration and in the function definition are different, and
you want them to be the same 'n'.
Erik
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