Re: Discriminants of tagged types

[Maciej Sobczak <see.my.homepage@gmail.com>]

On 1 Lis, 01:44, Shark8 <onewingedsh...@gmail.com> wrote:

> The assignability of PostScript objects is, IMO, a non-issue.
> Most objects are created via PostScript commands

Which makes them good candidates for being limited, as objects of
limited types can be initialized by functions.

On the other hand, limited types don't work with containers, so you
might want to consider that as a trade-off.

> > In the tutorial analogy of famous shapes, it might be perfectly
> > reasonable to duplicate an arbitrary Shape'Class (whatever it is), but
> > it will be completely unreasonable to assign Square to some existing
> > Circle.
>
> Interesting you mention that; I was working on a bit of a vector-
> graphic program for a computer graphics class and I was kicking around
> the idea of allowing such 'assignments' as being the equivalent of
> "make a shape of this new type which has the same bounding rectangle."
> I eventually decided against it because:
> 1 - The existence of an arbitrary and complex polygon would gum up the
> works,
> and
> 2) I'm lazy.

Laziness is a very good reason ;-), but in this case I think what is
more important is the fact that such assignment semantics would be
arbitrary (what about assigning colors?) and not self-explanatory at
the usage site.
Compare these two:

A := B;  -- what's going on here?

A.Resize_And_Move_For_New_Bounding_Rectangle (B);

I leave it up to you to invent a better operation name, but you get
the point.

I have never implemented any PostScript interpreter, but judging from
what you have described, I would make everything limited without much
regret.

--
Maciej Sobczak * http://www.inspirel.com