[Haskell-cafe] Question on rank-N polymorphism

[Vladimir Reshetnikov]

Hi Zsolt,

fs :: (((a, a) -> a) -> t) -> (t, t)
fs g = (g fst, g snd)
examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs
((,)id), fs (:[]), fs repeat)

    No instance for (Num [Char])
      arising from the literal `1' at M.hs:6:54
    Possible fix: add an instance declaration for (Num [Char])
    In the expression: 1
    In the second argument of `($)', namely `(1, "2")'
    In the first argument of `fs', namely `($ (1, "2"))'

Anyways, this signature is not what intended. I want it to work for
all tuples, regardless of their element types.

Thanks
Vladimir

On 6/7/09, Zsolt Dollenstein <zsol.zsol at gmail.com> wrote:
> On Sun, Jun 7, 2009 at 9:17 AM, Vladimir
> Reshetnikov<v.reshetnikov at gmail.com> wrote:
>> Hi Zsolt,
>>
>> It does not compiles with GHC without type annotations.
>
> It does with mine: The Glorious Glasgow Haskell Compilation System, version
> 6.10.2
>
> Anyway, try this: fs :: (((a, a) -> a) -> t) -> (t, t)
>
>>
>> Thanks,
>> Vladimir
>>
>> On 6/7/09, Zsolt Dollenstein <zsol.zsol at gmail.com> wrote:
>>> Hi Vladimir,
>>>
>>> On Sun, Jun 7, 2009 at 12:06 AM, Vladimir
>>> Reshetnikov<v.reshetnikov at gmail.com> wrote:
>>>> Hi,
>>>>
>>>> I have the following code:
>>>>
>>>> --------------------------------------------------------------------------------
>>>> fs g = (g fst, g snd)
>>>> examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs
>>>> ((,)id), fs (:[]), fs repeat)
>>>> --------------------------------------------------------------------------------
>>>>
>>>> The idea is that fs accepts a polymorphic function as its argument.
>>>> What type signature can I specify for f in order to compile this code?
>>>
>>> Have you tried putting the above into ghci for example, then asking for
>>> ":t
>>> fs"?
>>> Or am I misunderstanding your point?
>>>
>>> Cheers,
>>> Zsolt
>>>
>>>> If it is not possible in Haskell, is there another language with
>>>> static typing which allows this?
>>>>
>>>> Thanks,
>>>> Vladimir
>>>> _______________________________________________
>>>> Haskell-Cafe mailing list
>>>> Haskell-Cafe at haskell.org
>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>>
>>>
>>
>
>