[Haskell-cafe] OOP'er with (hopefully) trivial questions.....

Jules Bean jules at jellybean.co.uk
Tue Dec 18 07:30:43 EST 2007

Felipe Lessa wrote:
> On Dec 18, 2007 7:51 AM, Jules Bean <jules at jellybean.co.uk> wrote:
>> class Shape a where {
>>    intersect :: Shape b => a -> b -> Bool
>> }
> Shouldn't this be
> class Shape a where
>   ....whatever....
> class (Shape a, Shape b) => Intersectable a b where
>   intersect :: a -> b -> Bool
> With your definition I don't see how you could make it work, as you
> would have to write a function that takes care of this shape
> intersecting with any other shape, but this is exactly the problem the
> classes should solve!

Yes, that's a better solution, certainly. MPTCs are not haskell though 
:P I'm half joking, but there are solutions which don't involve 
non-standard extensions even ones as popular as MPTCs.

I didn't really think mine was particularly useful, just pointing out 
the design space, and in particular the precise parallel between the 
classes approach and the explicit dictionary approach.


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