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Mastering Modern Perl: Writing Efficient and Maintainable Code

Problems and Uses of Prototypes

Explore the complexities and appropriate uses of Perl prototypes, including how they affect code parsing and argument handling. Understand when to avoid them and how to apply them effectively for overriding built-ins, creating compile-time constants, extending syntax, and customizing functions.

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The problem with prototypes

Prototypes change how Perl parses our code and how Perl coerces arguments passed to our functions. While these prototypes may superficially resemble function signatures in other languages, they’re very different. They don’t document the number or types of arguments functions expect, nor do they map arguments to named parameters.

Prototype coercions work in subtle ways, such as enforcing scalar context on incoming arguments:

C++
sub numeric_equality($$) {
    my ($left, $right) = @_;
    return $left == $right;
}
my @nums = 1 .. 10;
say "They're equal, whatever that means!" if numeric_equality @nums, 10;

But they work only on simple expressions:

Perl
sub mypush(\@@);
    # compilation error: prototype mismatch
    # (expects array, gets scalar assignment)
mypush( my $elems = [], 1 .. 20 );
say "$elems";

To debug this, users of mypush must know both that a prototype exists and the limitations of the array prototype. That’s a lot of cognitive burden to put on a user—and if you think this ...