Assigning default values to function arguments which may be "empty"

Question

In perl, is there a canonical practice or idiom for assigning value to an argument which, if "empty", is assigned a default value?

Here, "empty" is defined as an argument value which is either undef or the null string (''). Notably, 0 (or perl false) is a valid, non-"empty" value, which rules out the construction $min_perl_version = (shift ||= '5.008009').

This is my current construction (note the use of the v5.10+ logical-or operator, //):

my $min_perl_version = do {$_ = shift // q{}; (length) ? $_ : '5.008009'};

as in the example function:

sub is_CORE { # ( $:MIN_PERL_VERSION , $:MODULE )
 use 5.008009; # perl v5.8.9; required for Module::CoreList to be available in CORE
 my $min_perl_version = do {$_ = shift // q{}; (length) ? $_ : '5.008009'}; # "empty" => default to version 5.008009
 my $module = shift;
 return ($Module::CoreList::VERSION >= 2.99) && Module::CoreList::is_core($module, undef, $min_perl_version);
}

I don't want warnings or to duplicate the use of the default value.

Is there a simpler / more elegant solution, say... without a named temporary variable?

A succinct solution which is functional back to earlier perl versions (5.6 or 5.8) would be a plus as well.

edit: for clarity

For clarification, the function interface and definition of argument validity (eg, what "empty" means) are set constants. While I appreciate any critique, a conversation about the line:

my $min_perl_version = do {$_ = shift // q{}; (length) ? $_ : '5.008009'};

and how it might be better coded is the real point of the question.

• mpapec: Ok, nothing canonical about it; my ($arg) = @_; $arg = "default_value" if !length($arg // ""); eval.in/624512
• mpapec: For perl older than 5.10 eval.in/624514
• mpapec: @rivy np, you can ask this kind of question at stackoverflow.com/questions/tagged/perl
• rivy: I am not at all adverse to rewriting this to be more well-formed for the context of CR. I could pare the question down to show just that single line. But, to me, it's confusing when removed from the context of the enclosing function. Hence, I used a more generic surrounding function initially. When asked for a real-use example, this surrounding function is simply the smallest, easily available use that I found. I've tried to add clarification, and, again, I can rewrite it. I do think code review of a single line / implementation is valid. But tell me if this site is the wrong place for that.

Answer 1

use 5.008009 is insufficient here, since you're using the // operator, which was introduced in Perl 5.10.

This function feels awkwardly designed. Generally, parameters that have default values appear after all required parameters. Here, I would need to explicitly pass in undef as the first parameter to take advantage of the default. Also note that the parameters for your is_CORE are backwards relative to Module::CoreList::is_core.

I also don't understand the benefit of having the empty string trigger the default. I can understand having undef trigger the default. I could also agree with having all "falsy" values trigger the default. But this particular design feels weird. If I had to do it, though, I'd write it like this:

sub is_CORE {
 use 5.008009; # perl v5.8.9; required for Module::CoreList to be available in CORE
 my ($module, $min_perl_version) = @_;
 $min_perl_version = '5.008009' unless defined $min_perl_version && length $min_perl_version;
 $Module::CoreList::VERSION >= 2.99 && Module::CoreList::is_core($module, undef, $min_perl_version);
}

Answer 2

After some helpful discussion (notably from PerlMonks), I was led to this construction:

my $arg = List::Util::first { defined && length } ( shift, 'default_value' );

It's DRY. It doesn't introduce any extra temporaries. And, to my eyes, it's simpler, easier to understand, and just more elegant.

As a bonus, it's portable, without requiring installation of any extra modules, back to perl v5.7.3 (the first version including List::Util in perl CORE).

I would refactor the example function as:

sub is_CORE { # ( $:MIN_PERL_VERSION , $:MODULE )
 use 5.007003; # perl v5.7.3; required for List::Util to be available in CORE
 use 5.008009; # perl v5.8.9; required for Module::CoreList to be available in CORE
 use List::Util qw/ first /;
 my $min_perl_version = first { defined && length } ( shift, '5.008009' ); # "empty" => default to version 5.008009
 my $module = shift;
 return ($Module::CoreList::VERSION >= 2.99) && Module::CoreList::is_core($module, undef, $min_perl_version);
}