interfaces :-(

[sean.fritz at gmail.com]

In article <dqoho0$bmh$1 at digitaldaemon.com>,
=?ISO-8859-1?Q?Jari-Matti_M=E4kel=E4?= says...
>
>xs0 wrote:
>
>As far as I understand it, the current interfaces can only be used on
>simple single inheritance structures and on closed source products that
>want to hide the implementation from end users. At least I cannot come
>up with any creative ways in using them.

Well, FWIW I've been using invariant return type Java interfaces for years
without realizing it was even a problem.  I don't use abstract classes on a
regular basis, because I tend to practice inversion of control and other design
patterns that promote the use of interfaces as the only name coupling between
classes wherever possible.

I don't really, at this time understand the need for covariant return types.  My
first reaction to learning about them was to view them as needlessly complex,
mostly useful in order to save one or two pointer lookups at method dispatch
time in certain select situations.  They are nice in that they do allow for
compile time type checking, which leads into my second reaction.

My second reaction was that all of these issues should be solvable by
parameterizing(is that the correct D term for putting a type parameter on an
interface?) the interface  in a similar way to the way Comparable<T> is
parameterized in Java.  That is if D allows for overloading based on return
types (which I do not know the answer to).

While implementing parameter covariance through Comparable<T> has some serious
issues, most notably you can never write the following code, it is one possible
solution to this problem.

//example illegal code -- you can't implement the same interface with 
//different type arguments in Java

class A implements Comparable<A> {
compareTo(A o) {
..
}
}

class B extends A implements Comparable<B> { //compile time error B cannot
//implement Comparable<A> and
//Comparable<B> simultaneously
..
}

PS:  Sorry if these points have already been made.  I realize this is an old
post.  I just happened across this newsgroup and was reading from the beginning.

Sean Fritz