Re: Real OO (was Choice of OO primitives in Ada95)

[donh@syd.csa.com.au (Don Harrison)]

In article <Do7unJ.1sq@world.std.com>, bobduff@world.std.com (Robert A Duff) writes:
:In article <Dns8pv.BDx@assip.csasyd.oz>,
:Don Harrison <donh@syd.csa.com.au> wrote:
:>Various arguments have been advanced to justify co-encapsulation. Below is a
:>collective response.
:>
:>1) Co-encapsulation preserves the symmetry of operands.
:>
:>As indicated in a previous post, the requirements of polymorphism demand that
:>operands are intrinsically asymmetric which suggests that uni-encapsulation is
:>more appropriate.
:
:Correct when there are two tagged types involved (unless you have
:multi-methods, as in CLOS).
:
:However, the symmetry is *not* lost when you have something like:
:
:    procedure Union(X, Y: Set) return Set;
:
:In other OO languages, the parameter X is the special one, and symmetry
:is lost.  Not a big deal, I admit, but I find it slightly cleaner,
:conceptually, to consider X and Y to be on equal footing.

I agree this visually more symmetrical. Of course, the operation still 
semantically belongs to tagged type Set - it is a primitive operation of it.

:
:- Bob
:

Don.