Re: limited/non-limited in Ada95

[bobduff@world.std.com (Robert A Duff)]

In article <mheaney-ya023680001610972221540001@news.ni.net>,
Matthew Heaney <mheaney@ni.net> wrote:
>What "important capabilities" do limited types have? ...

The important capability of limited types is that you can't do
assignment.  Maybe you don't call that a "capability", but I think it
is.  Perhaps the right way to put it is: the programmer (of some
abstraction) has the capability to document the fact that no assignments
will happen on this type.  And one can trust that documentation, since
the compiler verifies it (as opposed to comments).

It's like asking, "What capability do constants have?"  After all,
variables are just like constants, but can do more things (like have
their value change).  So what's the point of constants?  Mainly, the
point is that the reader of the code knows "this thing won't change".
(There's also an efficiency point -- the compiler can take advantage of
the fact that so-and-so won't change.)

One could view "constant" as a restriction (you can't say "X := 3;") but
I think it's just the opposite -- the Ada 'constant' feature allows the
programmer to express something useful.  There's no restriction -- if
you want to change it, just say so (ie just don't make it a constant).

Likewise, for limitedness.  The fact that you can't assign to a limited
thing is not a restriction, but, on the contrary, allows the programmer
to express an interesting fact.  If you *want* to assign to it, fine,
don't make it limited.

- Bob