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

[Bastian Erdnüß]

On Nov 19, 2010, at 1:07, Daniel Peebles 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.

Or you put it in the type class as fromInt :: Int -> n and do in Zero: fromInt _ = Zero and in the other: fromInt _ = Succ $ fromInt (undefined :: n).  Shouldn't that work, too?

> 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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