Re: Using "with function"

[Richard Riehle <richard@adaworks.com>]

Matthew Heaney wrote:

> Also, in general, you should use default notation for generic formal
> subprograms, like this:
>
> generic
>    type Element_Type is private;
>    with function "=" (L, R : Element_Type) return Boolean is <>;  --say "is
> box"
> package GP is

It is nice to hear from Matthew Heaney.  He has been absent for far too
long from this forum, and we always enjoy his comments.

I am going to respectfully disagree with Matthew on this.   While using the
"box" makes it easier to instantiate the "with function" it also carries with
it some danger.   Both Booch and Ed Berard made the case many years
ago for avoiding the use of operators directly in generic formal function
parameters.   In the GRACE components it was common to see somethin
like this:

     generic
        type Element_Type is private;
        with function Is_Equal(L, R : Element_Type) return Boolean;
     package GP ...

This has the effect of demanding that the instantiating unit supply a
generic actual parameter.

I recall a PhD thesis from Ohio State (apologize for forgetting the
author's name) in which the case was made for making all generic
formal parameters limited private and requiring the instantiation
to provide all kinds of operations, including a copy procedure
since assignment cannot be overloaded.

Of course, now that we can create generic signature packages in
Ada 95, that is packages that contain only generic formal parameters
for further instantiation of generic formal package parameters, the
Ohio State thesis gains a bit more credibility.

Richard Riehle