Why Are Perl 5’s Function Prototypes Often Considered Bad?

In Perl, function prototypes allow you to define subroutines that resemble Perl’s built-in functions. While prototypes have their niche uses, many Perl developers advise caution—or even avoidance—when using them. This guide will walk through why Perl’s prototypes are often seen as problematic and discuss scenarios where they can still be useful.

What Are Perl Prototypes Supposed to Do?

Unlike prototypes in many other languages, Perl’s function prototypes don’t perform compile-time argument checking. Instead, their main purpose is to allow user-defined functions to mimic built-in functions in terms of syntax and behavior. This means that prototypes primarily:

• Allow you to omit parentheses when calling the function.
• Impose specific contexts (e.g., scalar or list) on the arguments.

Here’s a simple example:

sub mypush(\@@) {
    my ($array_ref, @items) = @_;
    push @$array_ref, @items;
}

my @array;
mypush @array, 1, 2, 3;  # Works without requiring parentheses

In this example, mypush is defined with a prototype (\@@), allowing it to be called like a built-in push, without explicitly referencing the array with \.

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