I'm sure i shouldnt say this, but more than once I have written

if ( !$x == !$y ) { ... }
if ( !$x != !$y ) { ... }
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which I think is a fair exception to your rule. I kinda view ! and !! as "boolean constructors", and so long as both sides of your (in)equality are guaranteed to be a "boolean" obtained one of these constructors you can compare them.

But I'm really just nit-picking. :-)

There's a logical operator for that: xor

if ( !$x == !$y) ) # bool($x) == bool($y)
if ( !$x != !$y  ) # bool($x) != bool($y)
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can be written as

if ( !($x xor $y) ) # bool($x) == bool($y)
if (   $x xor $y  ) # bool($x) != bool($y)
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Unforunately, both sets have readability issues.

In Perl 6 we can use positive boolean context, which helps readability a little:

if ?$x == ?$y {...}
if ?$x != ?$y {...}
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We can use junctions, which again helps the different case more than the same case:

if not $x ^ $y {...}
if $x ^ $y {...}
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or if you prefer:

if !one($x,$y) {...}
if one($x,$y) {...}
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That's probably enough ways to do it...

Ah yes, scalar xor, i always forget about it. Personally the !$x == !$y makes more sense to me than !($x xor $y). But whatever.

---
$world=~s/war/peace/g

Even outlawing comparisons to the constants isn't enough.

sub is_tested {
    return FALSE;
}
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and then used as

  if( is_tested() ) {
     print "is tested.\n";
  }
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and, by the way, FALSE is defined to be 2. This was the sort of thing that haunted me for days.

I was using those expressions to describe the problem not the usage.

Wow, defining your FALSE constant to a value that isn't actually false is such an amazingly bad idea that I never even considered it. Yes, don't define Boolean constants that completely defy your language's concept of "true" or "false". Duh! :) Thanks for mentioning that.

- tye        

I have often said that I have learned as much from the really bad code that I have maintained as I have from the really good code I have maintained. (That applies to both my code and other people's code.)

If a value of FALSE that is not false doesn't scare you enough, I won't tell you what I found FOUR defined as...<grin/>

G. Wade