Re: disjoint ranges ?

[Lao Xiao Hai <laoxhai@ix.netcom.com>]

Philippe Torres wrote:

> A very dumb question but i can't find the right syntax so here i am.
> How can i declare a type or subtype with a disjoint range ?
>
> Example :
>
> If i declare :
> ---
> type Hours is new Integer range 0 .. 23 ;
> --
> i would like a
> ---
> subtype Lunch_Hours is Hours range 7..9 & 12..14 & 19..21 ;
> ---

The problem of disjoint ranges came up for me many years ago on a different
project.   I
even suggested this as a capability for Ada 9X, but my suggestion either
fell through the
cracks or I submitted it incorrectly.

My first attempt at a solution was to try to overload the "in" test as a
boolean function.  That
turns out to be illegal for a variety of reasons that require careful study
of the language rules
for the "in" test.   It turns out that membership tests are not
operations/operators in Ada.

I finally ended up declaring a function that would do this test, but did
not use operator
overloading.   It does solve the problem, but requires you call the
function each place
you intend to use a value of the type.

A better approach  would probably be to add a capability for assertion
checking.
This simple mechanism could be added at the type level or the subprogram
level
within the package specification.  The notion of range checks was a good
one in
the early design of Ada.   Perhaps the same thing can be accomplished
through
assertions in some future version.

Richard Riehle