David Feuer writes:

1. Why can't [instances] be hidden in module imports/exports?

The way I see it, an instance declaration is an assertion that a certain data type supports a certain set of operations. Thus, if the data type and the operations on it are in scope, it makes sense for the class instance to be, too. (This leads to the question of why we need to have instance declarations at all :-) (My guesses would be: compiler implementation issues, code clarity, error detection, partially implemented classes)) Problems arise when a data type needs to be instantiated twice in the same class, but with different operator implementations. (I.e. you have a data type which prints differently according to which module you're in) This would, I think, be a problem in most languages, think of deriving a C++ class twice from the same base class while providing different overrides for the functions. I'm not entirely convinced it's an issue that needs a better resolution than the language provides today. (There's a diminishing returns effect when adding language features, and at some point, the increased complexity of the language doesn't make it worth it, IMHO.)

2. Why can't you simultaneously declare a type to be an instance of multiple classes?

Why does it matter?

class C1 t where a::t->t class C1 t => C2 t where b::t->t instance C1 T, C2 T where a=... b=...

To me, it'd make more sense to provide class C1 t where a :: t -> t class (C1 t) => C2 t where b :: t -> t instance C2 T where a = ... -- implicitly instantiating C1 b = ... and avoid long instantiation chains. But that too is IMHO a minor issue. -kzm -- If I haven't seen further, it is by standing in the footprints of giants