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From: dewar@cs.nyu.edu (Robert Dewar)
Subject: Re: DECAda/VMS - calling GETJPI
Date: 1996/06/03
Message-ID: <dewar.833818928@schonberg>#1/1
X-Deja-AN: 158251547
references: <31B2AF74.668E@dial.eunet.ch> <4ouo97$pe8@gcsin3.geccs.gecm.com>
 <4ouuuv$6hd@linus.mitre.org>
organization: Courant Institute of Mathematical Sciences
newsgroups: comp.lang.ada
Date: 1996-06-03T00:00:00+00:00
List-Id: <comp.lang.ada>


Micheal Brenner said:

"you could just assign it:

  integer_minus_1: integer := -1;
  x: unsigned_longword := unsigned_longword (integer_minus_1);

because Ada 95 modular types truncate numbers to their modular range
without giving a constraint_error."

That's quite wrong, the type conversoin checks that the value is in
range, and the value is outside the range. Indeed if you add a constant
to the declaration of integer_minus_1, the out of range condition will
be detected at compile time by GNAT:

     1. procedure z is
     2.   type ul is mod 2 ** 64;
     3.   integer_minus_1: constant integer := -1;
     4.   x: ul := ul (integer_minus_1);
                       |
        >>> warning: static value out of range of type "ul" defined at line 2
        >>> warning: "constraint_error" will be raised at runtime

     5. begin
     6.    null;
     7. end;

Yes, you can apply the unary minus operator to an unsigned vaue and get
modular results as expected, but here the minus is applied to a signed
type and generates a real minus one, which is definitely outside the
range of any unsigned type.

It's probably time again to repost the plea that if you suggest a solution
to a problem, compile an example and make sure it works. No one is realiable
enough to be 100% accurate without such a backup check, and posting incorrect
information on Ada can cause a lot of confusion!

IN fact the proper solution in this case is trivial:

procedure z is
  type unsigned_longword is mod 2 ** 64;
  x: unsigned_longword := -1;
begin
   null;
end;

or, if you want the constant integer_minus_1, define it as type ul to start
with.