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

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