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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: GNAT messages and the not operator (pitfall alert!)
Date: 1996/10/24
Message-ID: <dewar.846162354@merv>#1/1
X-Deja-AN: 191758115
organization: New York University
newsgroups: comp.lang.ada
Date: 1996-10-24T00:00:00+00:00
List-Id: <comp.lang.ada>


Here is an entertaining little bit of error message stuff in GNAT. Someone
submitted the following program, shown with its original message:

     1. procedure t is
     2. begin
     3.     if not 7 < 5 then
               |
        >>> no modular type available in this context

     4.         null;
     5.     end if;
     6. end;

Well, that error message is "correct", the expression "not 7" cannot
be resolved. But it sure is confusing. The latest version of GNAT (3.08)
gives:
     1. procedure t is
     2. begin
     3.     if not 7 < 5 then
                   |
        >>> operand of not must be enclosed in parentheses

     4.         null;
     5.     end if;
     6.
     7.     if (not 7) < 5 then
                |
        >>> no modular type available in this context

     8.         null;
     9.     end if;
    10.
    11. end;

I have added the parenthesized case here to make sure we do the right thing!

Now, a worse case, suppose that we *are* dealing with modular types, well
then the extension of Ada 95 to allow not on such types has added a real
pitfall to the language. Ada does not have many pitfalls. I mean by pitfall
a case where you write something that looks reasonable, is legal and executes
but does something completely different from what you expect (e.g. in
C the case of if ("abc"=="abc") yielding false).

Consider this program, which we now, as you see, generate a warning for:

     1. with Text_IO; use Text_IO;
     2. procedure t1 is
     3.    type m is mod 256;
     4.    a : m := 4;
     5.    b : m := 5;
     6. begin
     7.     if not a < b then
               |
        >>> warning: operand of not operator should be parenthesized

     8.         Put_Line ("this is what I expect");
     9.     else
    10.         Put_Line ("but this is what I get!");
    11.     end if;
    12.
    13.     if (not a) < b then
    14.         Put_Line ("this is not what I expect");
    15.     else
    16.         Put_Line ("this is what I get, and I really asked for it!");
    17.     end if;
    18. end;

I think this warning is quite reasonable, since I would *really* like people
to use parens if they really want the situation shown on line 13!