Re: initialize an array (1-D) at elaboration using an expression based on the index?

[Adam Beneschan <adam@irvine.com>]

On Nov 10, 6:33 am, Georg Bauhaus <rm.dash-bauh...@futureapps.de>
wrote:
> On 16.10.10 02:16, Adam Beneschan wrote:
>
> > It seems like it would be simple to add syntax to array aggregates in
> > the language like
>
> > a : array(1..N) of Float := (for I in 1..N => Float(I)*Float(I));
>
> I have an array of library level task objects.
> Each task is supposed have a unique identifying number,
> fixed and assigned at compile time.  Perhaps starting from
> a task type definition like the following one:
>
>    task T (Identity : Job_Id := No_Job) is ...
>
> Could some variation of the (for ...) expression solve this,
> i.e. provide the initial value?
>
>    Jobs : array (Job_Id) of T := (for D in Job_Id => ???(D));
>
> Borrowing from limited types, I'd think "???" might be just "T"?
>
>    Jobs : array (Job_Id) of T := (for D in Job_Id => T (D));

This seems like an orthogonal problem.  That is, it's a problem even
when arrays aren't involved.  If you have a record that contains a
task---say you want to create a linked list:

   type Rec;
   type Rec_P is access all Rec;
   type Rec is record
      Job  : T;        -- T is defined with a discriminant
      Link : Rec_P;
   end record;

and you want to create an object of this type and initialize it with
an aggregate:

   Single_Job : aliased Rec := (T(J), null);

where J is some variable or parameter or something, I don't think
there's any current syntax that would allow this.  But I think you can
set up a function:

   function Task_With_ID (Identity : Job_ID := No_Job) return T is
   begin
      return X : T(Identity);
   end;

and use that in your aggregate:

   Single_Job : aliased Rec := (Task_With_ID(J), null);

Warning: I have not tried this.

If this does work, then assuming that a "for" syntax is added for
array aggregates, it can be used in conjunction with a function call
as above.  If you don't like the need for a function call and would
like a different syntax, then one can be proposed, but any solution
should also work in the non-array case.  So this doesn't really have
anything to do with array initialization.

                           -- Adam