Re: default formal parameters in generic declarations

[Stephen Leake <stephen_leake@stephe-leake.org>]

Eric Hughes <eric.eh9@gmail.com> writes:

> Eric Hughes <eric....@gmail.com> writes:
>> I would like to implement this in Ada, but the absence of a default
>> package parameter means that instantiation is horribly clunky and that
>> only the most dedicated would use it.
>
> On Mar 3, 5:42 am, Stephen Leake <stephen_le...@stephe-leake.org>
> wrote:
>> I don't see why this would be "horribly klunky"; they typical user
>> will do:
>
> For a single package instantiation, a default formal package would
> have the same benefit as a default formal subprogram.  It removes from
> a programmer's mind a detail that may not be relevant, and it removes
> from a less-experienced such mind a barrier to entry.

Right.

> I said "horribly klunky" specifically in reference to instantiation of
> aspects.  The problem arises because, in general, an aspect _ought_ to
> have access to the internal state of the package for which it is
> acting as an aspect.  

Right. That's one reason I don't think aspect-based programming is all
that useful.

> That means that a base package requires an aspect parameter to
> instantiate it at the same time as the aspect requires a base
> package to instantiate it. The hard problem I had to figure out was
> how to avoid infinite regression. 

Ok.

> In C++, the solution was to rely upon a detail of template
> instantiation where a self- reference to a class itself with the
> template parameters of its instantiation refers _not_ to the
> templated base class but to the class being instantiated, and so
> thus creates (de facto) an anonymous type whose introduction breaks
> the regression.

Ok. Did you find a similar mechanism in Ada yet? Does limited with help?

> If that sounds complicated to understand, it is.  The minimal syntax
> to declare and to instantiate is not simple.  In fact, it's really
> quite a mess, and I couldn't figure out how to simplify it (even more--
> I have some idea that it's minimal).  Requiring an ordinary user to
> learn even some this detail simply to use the class in its ordinary
> way is a poor information-hiding practice.

Ok. I can see that this might justify "horribly klunky".

-- 
-- Stephe