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

[Job Vranish]

I think what you probably want is something like this:

class Chunckable c where
  cLength :: c el -> Int
  cHead :: c el -> Maybe el
  cMap :: (a -> b) -> c a -> c b

instance Chunckable [] where
  cLength [] = 0
  cLength (x:xs) = 1 + cLength xs

  cHead [] = Nothing
  cHead (x:xs) = Just x

  cMap = map

a = [4, 7, 3, 8]
test1 = cLength a
test2 = cHead a
test3 = cMap (Just) a


The class does not actually need the second type parameter.
You can actually use all sorts of extra type variables in the type
signatures in class declarations as long as all your instances are
polymorphic across those types (not sure if that's the precise
terminology). Basically, as long as cLength, cHead and cMap do the
same thing regardless of what el is, then you don't need to have el as
a type parameter to the class.

Now if you _do_ want to have cLength, etc do something else depending
el, then things get more complicated. Maybe something like this:

class Chunckable2 c el where
  cLength2 :: c el -> Int
  cHead2 :: c el -> Maybe el
  cMap2 :: (Chunckable2 c el') => (el -> el') -> c el -> c el'

instance Chunckable2 [] Int where
  cLength2 [] = 0
  cLength2 (x:xs) = 1 + cLength xs

  cHead2 [] = Nothing
  cHead2 (x:xs) = Just x

  cMap2 = map

instance Chunckable2 [] Float where
  cLength2 [] = 0
  cLength2 (x:xs) = 1 + cLength xs

  cHead2 [] = Nothing
  cHead2 (x:xs) = Just x

  cMap2 f xs = []

test4 = cMap2 (fromIntegral) (a::[Int]) :: [Float]
test5 = cMap2 (id) ([3.0, 4.0, 1.0]::[Float])  :: [Float]

Note that if you want things to work like this, functional
dependencies wont help you out (as they don't make sense in this case)



On Wed, Feb 11, 2009 at 12:34 PM, John Lato <jwlato at gmail.com> wrote:
> 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
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
>>>
>>
>