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From: "Mark A. Biggar" <mark@biggar.org>
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Newsgroups: comp.lang.ada
Subject: Re: Overlapping ranges
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Date: 2003-06-22T01:23:56+00:00
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AG wrote:
> Assuming I have a package declaration like this:
> 
> 
> package range is
>   type x1 is range 0..255;
>   type x2 is range 127..2**15-1;
> end ranges;
> 
> 
> Is there a way to declare a function
> using that package that will accept
> values of either x1 or x2 (and nothing
> else) restricted to the range 127..255 ?

You have a bug in your design abstraction.  x1 and x2 are different
types and there is no way to declare a parameter with more then one
type.  This is like the doctor joke: "patient: Doc it hurts when I do 
this.  Doctor: So don't do that!"

But if that's really what you want to do this, there are several way to
do it.

First option fix the abstraction:

package range is
    type xBase is range 0..2**15-1;
    subtype x1 is xBase range 0..255;
    subtype x2 is xBase range 127..2**15-1;
    subtype x1x2Intersect is xbase range 127..255;

    function foo(p: x1x2Intersect) return ...;
  end ranges;

If you really need two separate types:

package range is
    type xBase is range 0...2**15-1;
    subtype x1x2Intersect is xBase range 127..255;
    type x1 is new xBase range xBase'First..255;
    type x2 is new xBase range 127..xBase'Last;

    function foo(p: x1x2intersect) return ...;
end ranges;

But now you must call foo with a view conversion to xBase:
a := foo(xBase(200));

Another option is to use a discriminated record:

package range is
    type x1 is range 0..255;
    type x2 is range 127..2**15-1;

    type x1x2intersect(b: boolean) is record
	case b is
	when false => x1val: x1 range 127..255;
	when true  => x2val: x2 range 127..255;
	end case;
    end record;

    function foo(p: x1x2intersect) return ...;
end ranges;

But now you need to call foo like:
a := foo((false, x1'(200)));
or
a := foo((true,  x2'(200)));


Personally, I'd try to get the design fixed :-)

-- 
mark@biggar.org
mark.a.biggar@attbi.com