[Haskell-cafe] About "Fun with type functions" example

[Arnaud Bailly]

Just after hitting the button send, it appeared to me that fromInt was
not obvious at all, and probably impossible.
Not sure I understand your answer though: What would be the second
parameter (forall n . (Nat n) => n -> r) -> r) ?

Thanks
Arnaud

On Fri, Nov 19, 2010 at 1:07 AM, Daniel Peebles <pumpkingod at gmail.com> wrote:
> The best you can do with fromInt is something like Int -> (forall n. (Nat n)
> => n -> r) -> r, since the type isn't known at compile time.
>
> On Thu, Nov 18, 2010 at 2:52 PM, Arnaud Bailly <arnaud.oqube at gmail.com>
> wrote:
>>
>> Thanks a lot, that works perfectly fine!
>> Did not know this one...
>> BTW, I would be interested in the fromInt too.
>>
>> Arnaud
>>
>> On Thu, Nov 18, 2010 at 8:22 PM, Erik Hesselink <hesselink at gmail.com>
>> wrote:
>> > On Thu, Nov 18, 2010 at 20:17, Arnaud Bailly <arnaud.oqube at 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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