[Haskell-beginners] Re: [Haskell-cafe] About "Fun with type
functions" example
Jonas Almström Duregård
jonas.duregard at chalmers.se
Fri Nov 19 01:42:15 EST 2010
Then you can write fromInt 1 :: Zero, and it will type check.
/J
On 19 November 2010 04:42, Bastian Erdnüß <earthnut at web.de> wrote:
>
> 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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