Re: Proposed change to BC iterator parameters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Jeffrey Carter <spam@spam.com> writes:

> If we had a syntax like that I described, there would be no predefined
> "=" for either kind of type.

Then how do you write the body of the user-defined "="?

> One can think about all kinds of constraints on types, such as the
> integer types you mentioned. It would be nice to have unconstrained
> arrays with a fixed lower bound*:
> 
> type String_From_1 is array (Positive range 1 .. <>) of Character;

Indeed.  I think the way to do that would be to eliminate the "range <>"
syntax, and allow arrays types to have discriminants.  So the index
subtype would be given as a name, or a "name range
expression..expression", where either expression can refer to a
discrimant.  Like this:

  type String(Last: Natural) is array (Positive range 1 .. Last) of Character;

Instead of X'Last, one would say X.Last.  This seems simpler than having
special syntax "range <>", and then extending that to "range 1..<>"
and "range <>..10".

Something like that was proposed during the Ada 9X project.

> I don't expect to see major changes of this sort.

No, I don't expect that particular change in the next Ada.

>... Where do you stop?

You don't, I suppose.  Fortran is much older than Ada, and they keep
coming out with major revisions of Fortran.

> Should you allow arrays with a fixed upper bound?

Of course.  To disallow that would be an arbitrary restriction,
not a simplification (presuming you're going to allow fixed
lower bounds).

> * Ada sort of has this:
> 
> type String_From_1 (Length : Natural) is record
>     Value : String (1 .. Length);
> end record;

Sort of.  The syntax for string literals and slices becomes pretty awful.

- Bob