On 18-Feb-2000, Brian Boutel <[EMAIL PROTECTED]> wrote:
>
> OK, I understand now. But what advantage does explicit control of instance
> import/export give you over the current Haskell rule?
It's just a question of information hiding.
It lets you declare a type to be an instance of a public type class
without exporting that fact (and hence exporting those methods).
> In Haskell, you can have at most 1 C-T instance, and that is visible in
> every module in the program which can be reached by a chain of
> import/export declarations from the declaring module.
> In Mercury you can have at most 1 C-T instance, but there may be more
> places where it is not visible. There does not seem to be anything you can
> do in those places that you could not do if the C-T instance were visible
> there.
You could say the same about e.g. abstract data types.
The advantage is that the author of module A can ensure that
module B will not access the private parts of module A.
Certainly there's nothing more you could do in module B
than if those parts were public, indeed what you can do is strictly
less. But the whole point of making things private is to *restrict*
what other modules can do with them.
--
Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh> | of excellence is a lethal habit"
PGP: finger [EMAIL PROTECTED] | -- the last words of T. S. Garp.
