[Haskell-beginners] Multi-parameter type classes

[David Beacham]

Hi all,

I've been reading through Okasaki's purely functional data structures 
and noticed that the source he provides for the Set implementations uses 
multi-parameter type classes, whereas those for the Heap structures does 
not.

class Set s a where
   empty :: s a
   insert :: a -> s a -> s a
   ...

class Heap h where
   empty :: Ord a => h a
   insert :: Ord a => a -> h a -> h a
   ...

The only difference I can see is that the instance declarations for Set 
contain the Ord constraint, whereas they are defined on the class 
functions for Heap.

instance (Ord a) => Set RedBlackSet a where
    ...

I realise that the type of `a` needs to be constrained to Ord instances 
at some point in the code, but is there any particular reason for doing 
it differently in each case? If not, should one method be preferred over 
the other?

Thanks for the help, David.