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

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 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 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 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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe