in reply to if variants

One thing that's worth mentioning is that while this: 
if($foo == 7) {
    print "foo is 7\n";
} else {
    print "foo is not 7\n";
}

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is acceptable, this: 
unless($foo != 7) {
    print "foo is 7\n";
} else {
    print "foo is not 7\n";
}

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is not. Sure, perl will accept it (I consider this to be a bug in perl), and it'll run, but anyone writing it should be beaten savagely. It's VERY hard to read.

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Re^2: if variants
by apotheon (Deacon) on Oct 20, 2004 at 18:57 UTC
    I edited to include some brief mention of the undesirability of the != operator there. Thanks for the suggestion.

    - apotheon
    CopyWrite Chad Perrin
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      It's worth noting, by the way, that using $foo != 7 as the control statement in an if statement is generally a Bad Idea. The unless statement is designed to cover that need, and using the != operator there just makes the code harder to read later when it must be maintained.

      I second DrHyde (Re^3: if variants). It's definately not bad and neither is it a maintenance trouble. That whole sentence above should be removed from your text. It's just plain wrong.

      The unless keyword is made to make you write code more like a natural language. It's not there to take !='s place in if expressions. They can both coexist happily and be mixed however you please.

      Why the combination of unless and else is shunned by many is because it's a sort of double negation. 

      if (X) { ... } else { ... }

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      can be read 
      if (X) { ... } if (not X) { ... }

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      "unless X" can be read "if not X". Combining this gives you 
      unless (X) { ... } else { ... }
if (not X) { ... } if (not not X) { ... }

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      which just doesn't read well. Logically it's not strange but it's a funny and inappreciated way of expressing oneself.

      My own (rather inconsistent) style is to use unless for exceptional behaviour, and if for expected/wanted behaviour. When using unless I usually want something, but something might stop me from doing that. With if I express that I perhaps want something, perhaps not. 

      print ... unless $quiet; # I want to print.
exit if $done;           # I might want to exit here.
exit unless $stay;       # I want to exit here, but apparently
                         # something is holding me back.

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      The examples can perhaps be better, and I'm not consistant myself in my use, but that's give you an idea of how I tend to use them.

      ihb

      See perltoc if you don't know which perldoc to read!
      Read argumentation in its context!

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        Thanks for the clarification. Mea culpa.

        - apotheon
        CopyWrite Chad Perrin
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      Noooooo! != is fine. It's unless ... else that is hideously broken!
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        I can understand avoiding the != in an if statement, since using unless covers that functionality. I don't know why unless . . . else would be "bad", however. Explain?

        - apotheon
        CopyWrite Chad Perrin
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