Re: Iterable container as generic parameter

[Jere <jhb.chat@gmail.com>]

On Saturday, January 27, 2018 at 6:38:43 AM UTC-5, Lionel Draghi wrote:
> Le samedi 27 janvier 2018 08:03:49 UTC+1, Randy Brukardt a écrit :
> > ...
> > >By the way, I have another question. There is no way to provide a default
> > > instantiation to a formal package parameter?
> > 
> > Not in Ada 2012 or before. There is an open issue (it is one of those that 
> > Emmanuel referenced in his message) to allow that. It hasn't progressed any 
> > significant amount at this time.
> 
> Very interesting indeed, to have lawyers and implementers view on a language change proposal.
> 
> From my user point of view, the "or use" proposal looks very good.
> It's simple (to write!), easy to read (much more in my opinion than "with Default_Type => ..." syntax).

I agree.  I like the "or use" syntax best so far.

> 
> I would just add that, when reading the comment on the ":=" proposal, that ":=" may seems to imply copying a type, I realized that I was already reading :
> > procedure New_Line (Spacing : Positive_Count := 1);
> as "give your own spacing, _or use_ 1 if none given", and not as 
> "_copy_ 1 in Spacing if none given". 
> 
> So, IMHO, I wouldn't give a high weight to this argument.
>  
> And, actually, 
> > procedure New_Line (Spacing : Positive_Count or use 1);
> looks good too :-)
>  
While I don't like := as much as "or use", I will just add that
in today's Ada, := doesn't always imply copying.  It also can imply
build in place (somewhat similar to move semantics) in some cases.
So I agree that it isn't as much of a stretch.

I would still think "or use" is better though.

Another option that is less useful than default package types is
(making up a term here) package delegation/forwarding:

-- not legal Ada
with Specific_Package; use Specific_Package;

generic
   type Some_Type is limited private;
package Default_Package is new Base_Package
   (First_Param  => Specific_Thing,
    Second_Param => Some_Type);

I think you can do that or something similar in a generic formal,
but I don't think you can define a new generic like this.  It would
only be an alternative to default package formals (I would prefer
those), but would allow for defining a default package specification
for another.  It wouldn't be very useful if you wanted to default
a lot of things in different combinations though.