-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Johannes Waldmann wrote:
Henning Thielemann wrote: The above (local instance) could be modelled by handing the required Ord dictionary explicitely (or as an implicit parameter, which I would not recommend, though)
You can hand over the Ord dictionary when constructing a Set in Java:
TreeSet(Comparator comparator) Constructs a new, empty tree set, sorted according to the specified comparator.
So the Haskell collections could need a similar constructor? The difference to your work-around (Wrapper class for the elements of the collection) is that the constructor is called only once, but your wrapper clutters each and every access.
And then what do you do with (set1 `union` set2) where they have different comparators? C++ collections have that think-o, I don't know about Java.... the problem being that the comparators are not statically required to be "the same", just the same *type*, so, what do you do with two different ones? I suppose dependently-typed languages can directly express the equivalence requirement without requiring it to be statically determinable from the type system what the comparison operator is going to be. Compilers based on classes being functions from types to methods (jhc...) can't use the alternate-dictionary thing. But you can always directly have the (key -> key -> Ordering) function be part of your data structure for the same effect, and then you can see all the problems involved (serialization, interactions and comparisons between maps, etc.). If you just don't want to wrap and unwrap your *keys* all the time, perhaps you could have the statically checkable version: class Comparer comp where type ComparedType comp --associated type, not yet implemented anywhere compare' :: ~comp -> (ComparedType comp) -> (ComparedType comp) -> Ordering --the '~' means that that parameter must be unused (this is not an implemented syntax) (the parameter is only there to select which instance is being used, since you can't pass a type directly as an argument without an associated "value", in Haskell; rather, (undefined :: TheCorrectType) is passed) data NewMap comp k v = ...the same as Data.Map -- most operations on NewMap require the context (Comparer comp) data OrdComparer k --a "phantom" (data-)type with no constructors instance (Ord k) => Comparer (OrdComparer k) where type ComparedType (OrdComparer k) = k compare' = Prelude.compare type Map k = NewMap (OrdComparer k) k -- most operations on Map require the context (Ord k), which is a simplification of the required context (Comparer (OrdComparer k)) Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGJniXHgcxvIWYTTURAhNYAJ4+SWf6DO/gC0nLngopykIySKguQACfafdk kiD505V+pr2n2+BsOZS47Uo= =HTQ2 -----END PGP SIGNATURE-----