Re: disjoint ranges ?

[wayne lydecker <wlydeckr@pacbell.net>]

>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

How about using a boolean array:

with Text_IO;
procedure lunch is
  type Hours is new Integer range 0 .. 23 ;
  Lunch_Hours : array (Hours) of Boolean :=
    (7..9 => True, 12..14 => True, 19..21 => True, others => False);
begin
  if Lunch_Hours(7) then
    text_io.put_line("Lunch time");
  else
    text_io.put_Line("Get back to work");
  end if;
end;


-- Wayne.