Re: GNAT messages and the not operator (pitfall alert!)

[johnherro@aol.com (John Herro)]

dewar@merv.cs.nyu.edu (Robert Dewar) writes:
> Ask an Ada expert what is the value of
>    -5 mod 3
> Almost anyone [gets it wrong] ... the default
> parenthesiztion of this expression is -(5 mod 3).

This brings us to a coding style rule that I wish everyone would follow. 
Use parentheses unless the order of evaluaton is VERY OBVIOUS.  A person
reading your code should _never_ have to go to the RM to look up the
precedence of operators, and certainly you should never refer to that
section when writing code; use parentheses instead.

I'm not saying that the code

   if X**2 + 3.0*X + 2.0 < 100.0 then ...

should be cluttered with unnecessary parentheses.  Everyone knows that
"**" comes before "*" and "/", which come before binary "+" and "-", and
everyone knows that the relational operators have a low precedence.  It's
also arguable that "everyone" knows that "and" comes before "or".  But I'd
say in almost every other case, unless the order of evaluation is very
obvious, use parentheses!

Even Ada has some pitfalls.  If everyone followed this rule, one of them
would be no problem.

- John Herro
Software Innovations Technology
http://members.aol.com/AdaTutor
ftp://members.aol.com/AdaTutor