Is using prototypes to declare array reference con

Question

In the linked SO answer, Eric illustrates a way to call a subroutine, which accepts arrays by reference as arguments, and use the prototypes to allow the caller code to pass the array names without using reference operator \@; the way built-ins like push @array, $value do.

# Original code:
sub Hello { my ($x_ref, $y_ref) = @_; ...}
Hello(\@x, \@y);

# Same thing using array ref prototype:
sub Hello (\@\@$) {...}
Hello(@x, @y);

My question is, is this considered to be a Best Practice? And what are the guidelines on the pattern's use?

It seems like this pattern should either be used ONLY for built-ins, or for 100% of subroutines that accept array arguments in all of your code.

Otherwise code maintenance and use of your subs becomes fragile since the developer never knows whether a particular sub, when called, should be forced to reference an array or not.

An additional point of fragility is that you become confused between doing such calls and legitimately using two arrays combined into one using a comma operator.

On the positive side, using the pattern prevents the "forgot to reference the array" bugs, and makes the code calling the subroutines somewhat more readable.

P.S. I don't have Conway's book handy and don't recall if he ever discussed the topic, to pre-empt RTFB responses.

Answer 1

I tend to think that anything that makes a subroutine act differently than any other subroutine isn't the best thing. There might be instances where it's a good idea, but in general it's more special cases, documentation, and other things to remember. The more your code acts like most other Perl code, the easier you make things for your users.

Notice that this isn't the main complaint that Perl Best Practices has with prototypes, which is an ugly kludge in Perl.

Answer 2

Only use prototypes when you're trying to extend Perl's syntax: e.g. if you're building Moose, or something like the examples in Dominus' Higher Order Perl. If you're doing that, you know enough to disregard PBP (and to annotate your code to shut Perl::Critic up). If you're doing anything else, don't use them.

Answer 3

Let's ask Perl::Critic:

echo "use strict; sub Hello (\@\@$) {...}" | perlcritic
Subroutine prototypes used at line 1, column 1.  See page 194 of PBP.  (Severity: 5)

Yeah, that would be a no.