[Haskell-cafe] can't figure out a type

[John Lato]

Hi Job,

Thanks for answering.  What I'm trying to do is probably very simple,
and I think the biggest problem is that I don't fully understand kinds
yet.

Here's an example instance:

instance Chunkable [a] a where
  cmap = map
  --etc.

In the class I wrote, c has kind * (e.g. [a]), but then I don't see
how to write a suitable map function.  For that, I would want c to
have kind * -> *.  Unfortunately then I don't know to write the
others.

Would I have to do something with c having kind (* -> *) ?

class Chunkable2 c el where
   cLength :: c el -> Int
   cHead :: c el -> Maybe el
   cMap :: (el -> el') -> c el -> c el'

Sincerely,
John

On Wed, Feb 11, 2009 at 5:12 PM, Job Vranish <jvranish at gmail.com> wrote:
> What do you mean by parameterized over a different type?
> will c have a kind of * -> * ? I don't think it has to be for what you
> want to work, but the idea of "same instance" will go out the window.
>
> Do you have a small usage example?
>
>
> On Wed, Feb 11, 2009 at 11:52 AM, John Lato <jwlato at gmail.com> wrote:
>> Hello,
>>
>> I'm working on some code like the following:
>>
>>> class Chunkable c el | c -> el where
>>>   cLength :: c -> Int
>>>   cHead :: c -> Maybe el
>>
>> I want to be able to map over this type, like this:
>>
>>>  cMap :: Chunkable c' el' => (el -> el') -> c -> c'
>>
>> but this isn't quite right.  c' shouldn't be any instance of
>> Chunkable, it should be the same instance except parameterized over a
>> different type.  Another approach would be something like:
>>
>> class (Functor c) => Chunkable c el
>> ...
>>
>> except that's not right either.  I think c has the wrong kind to be a
>> Functor instance.
>>
>> I expect there's something very basic I'm missing.  Could anyone point
>> in the proper direction of how to do this?  Can this be expressed with
>> associated types, perhaps?
>>
>> Thanks,
>>
>> John Lato
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>