[Haskell-cafe] Newbie Q: Overloading and type classes

[Luke Palmer]

On Sat, Jun 7, 2008 at 1:08 PM, Luke Palmer <lrpalmer at gmail.com> wrote:
> 2008/6/7 Dmitri O.Kondratiev <dokondr at gmail.com>:
>> class Store s where
>>     put :: Eq k => (k, v) -> s  -> s
>>     get :: k s -> Maybe v
>
> I suspect you want this to be a constructor class.  That is, you want
> to make explicit the fact that the type s depends on k and v.
>
> class Store s where
>    put :: Eq k => (k,v) -> s k v -> s k v
>    get :: s k v -> Maybe v

Oops.  Should be:

    get :: k -> s k v -> Maybe v

And correspondingly for the later examples.  After actually using my
brain thinking about your problem, and reading the word "Newbie", I
would absolutely stay away from the fundeps/associated types business.
 :-)  Try to get this working with Cell and CellList first :-)

Luke

> If instead you have cell types which are restricted in what they can
> store in different ways, you might explore fundeps or associated
> types:
>
> -- fundeps
> class Store s k v | s -> k v where
>    put :: (k,v) -> s -> s
>    get :: s -> Maybe v
>
> -- associated types
> class Store s where
>    type Key s :: *
>    type Value s :: *
>    put :: (Key s, Value s) -> s -> s
>    get :: s -> Maybe (Value s)
>
> But if you can get away with the former, I would recommend that before
> looking into these advanced extensions.
>
> Luke
>