[Haskell-cafe] Instances of constrained datatypes

[Keean Schupke]

Can you not define functor like Hughes defines a restricted monad 
(section 3 in the paper)...

    Keean


Arjun Guha wrote:

>> One way to do roughly what you want is to pass the dictionary yourself:
>>
>>  >data EqDict a = EqDict {
>>   >        leq :: a -> a -> Bool }
>>   >
>>   >data EqList a = EqList (EqDict a) [a]
>>   >
>>   >test :: EqList a -> EqList a -> Bool
>>   >test (EqList dict (a0:as)) (EqList _ (b0:bs)) = (leq dict) a0 b0
>
>
> This is like the `object-oriented approach' in John Hughes' paper.  
> Let's switch to the set example in his paper:
>
> > data EqDict a = EqDict { isEq:: a -> a -> Bool }
> > data Set a = Set (EqDict a) [a]
>
> So, to make it a functor, as I originally wanted:
>
> > instance Functor Set where
> >   fmap f (Set dict ls) = Set dict' ls' where
> >     ls' = nubBy (isEq dict') ls
> >     dict' = ???
>
> There really isn't a way to define dict' for the resultant Set.
>
> -Arjun