Re: Question on modular types

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Jerry said

"Clearly, all values are invalid, and that without inporting any values
from the outside world."

No! If it is clear to you, you need your Ada prescription fixed :-)

   A := -1;

-1 is not a literal here, it is an expression, and the unary minus
operator is a modular unary minus. This is a useful, intended, and
clearly documented facility.

Note that if you did

    Minus1 : constant := -1;

    uint := minus1;

then you would get the constraint error:

     1. procedure b is
     2.    type x is mod 256;
     3.    k : constant := -1;
     4.    r : x;
     5.
     6. begin
     7.    r := -1;
     8.    r := k;
                |
        >>> value not in range of type "x" defined at line 2
        >>> static expression raises "constraint_error"

     9. end;

That's because the minus operator in line 3 is the one that works on
root integer (I would really prefer to say universal integer, but I
think root integer is correct), and really does give minus one, and
that is indeed out of range, as detected (statically in this particular
case).

Note that for modular types of sizes corresponding to hardware sizes
(typically 8,16,32) there will of course be no invalid values ever.