Recently I was writing a program to send me text messages when specific teams scored goals in the World Cup. Thanks to several monks who have already help me out with crontab problems :-)

Anyway, looking at some of the source code I noticed a very subtle bug. The program works as follows:

My program contained the code 
if ( ($team1_score ne $team1_scorec) or ($team2_score ne $team2_score)
+ ) {
    $changed_matches{$team1} = $changed_matches{$team2} = 
      "$team1 vs $team2 ($team1_scorec - $team2_scorec)"; 
  }

[download]
Obviously this code contains an error. In the second 'clause' of the 'if' statement, I'm comparing a variable to itself. I meant to write $team2_scorec instead.

So surely the expression $team2_score ne $team2_score is a contradiction--it will never evaluate as true.

So, wouldn't it be good if Perl could warn about this? It would have saved me a lot of trouble.

I suppose it might be possible that a thread could access the variable and change it, but I would imagine that the ne operator would be implemented in a sort of 'atomic' fashion.

I'd be interested to hear your views on this. Would it be difficult to create a pragma to spot contradictions and tautologies as well?

In reply to Should 'use warnings' check for contradictions? by SuperCruncher

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