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    <p>The problem is that Perl's function prototypes don't do what people think they do. Their purpose is to allow you to write functions that will be parsed like Perl's built-in functions.</p> <p>First of all, method calls completely ignore prototypes. If you're doing OO programming, it doesn't matter what prototype your methods have. (So they shouldn't have any prototype.)</p> <p>Second, prototypes aren't strictly enforced. If you call a subroutine with <code>&amp;function(...)</code>, the prototype is ignored. So they don't really provide any type safety.</p> <p>Third, they're spooky action-at-a-distance. (Especially the <code>$</code> prototype, which causes the corresponding parameter to be evaluated in scalar context, instead of the default list context.)</p> <p>In particular, they make it hard to pass parameters from arrays. For example:</p> <pre><code>my @array = qw(a b c); foo(@array); foo(@array[0..1]); foo($array[0], $array[1], $array[2]); sub foo ($;$$) { print "@_\n" } foo(@array); foo(@array[0..1]); foo($array[0], $array[1], $array[2]); </code></pre> <p>prints:</p> <pre><code>a b c a b a b c 3 b a b c </code></pre> <p>along with 3 warnings about <code>main::foo() called too early to check prototype</code> (if warnings are enabled). The problem is that an array (or array slice) evaluated in scalar context returns the length of the array.</p> <p>If you need to write a function that acts like a built-in, use a prototype. Otherwise, don't use prototypes.</p> <p>Note: Perl 6 will have completely revamped and very useful prototypes. This answer applies only to Perl 5.</p>
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    1. POWhy are Perl 5's function prototypes bad?
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    1. UScjm
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    1. COBut they still provide a useful check that your caller and the sub are using the same number of arguments, so what's wrong with that?
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      1. USPaul Tomblin
    2. COthat's sort of the point of my question - I never use the old &func syntax, so the prototype does provide useful function parameter checking.
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      1. USAlnitak
    3. CONo; the general consensus is that Perl function prototypes provide essentially no benefit. You may as well not bother with them, at least in Perl 5. Perl 6 might be a different (better) story.
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      1. USJonathan Leffler
 

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