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All the Perl that's Practical to Extract and Report

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  • No, I wouldn't use a switch-like construct there. What's the point of a label when there's a much cleaner solution? I would probably use a dispatch table like the following:

    my %dispatch = (
      foo => \&foo,
      bar => \&bar,
      baz => \&baz
    );

    #later

    if ( exists $dispatch{$function} ) {
      $dispatch{function}->(@args);
    }
    else {
      die "No such function ($function)";
    }

    I don't know, maybe it's just me, but that seems much cleaner and easier to read. Plus, even if you forget to test for the existence of the function in the dispatch hash, it will die immediately when you try to use an undefined value as a sub reference. I think that makes it less likely to be buggy.

    The only appropriate use of labels that I can think of is to solve thorny control flow problems for which other solutions would be less clear (as you demonstrate). Using them to make a switch statement is not one of those problems. I used to think so, but I found out how easy it is to write bugs in 'em.

    • Well, I use dispatch tables. But it hasn't caught on with most co-workers. Anyways, the point is trying to get a good way to explain to these people that the way they use labels is wrong. The examples I showed, although not the best way to do things (aside the nested loop), are more 'valid' ways to use labels. Dispatch tables are better than 'goto LABEL', but I show that as an example of a more proper usage. Basically, because people at my work seem to think that when I show them good programming practices,
      • I humbly submit an example of when SWITCH can go horribly wrong. The temptation to use labels for control flow when they are not required makes bugs like this more likely.

         while ( my $data = $t_sth->fetchrow_arrayref ) {
           my ( $amt, $id ) = @$data;
           $amt /= PRECISION;

           SWITCH: {
             $id == $CASH        && ($tcash        += $amt) && last SWITCH;
             $id == $ACCOUNT

        • Yes, I can try this one. I saw the bug when I saw the code. Maybe this one will show why they should avoid labels (although labels are good in deeply nested loops, they don't do that anyways).

          Maybe I can turn this into a "THAT IS FIRE, DO NOT TOUCH THE FIRE" ;-)

        • $id == $CASH && ($tcash += $amt) && last SWITCH;

          I saw the "bug" when I saw the code, but then I thought "the programmer wouldn't have used that construct if ($tcash += $amt) could ever yield zero!".

          I like pie.