PreshBlog

I often write Perl scripts which need to read options given on the command line. Normally I turn to the venerable old Getopt::Long which does the job.

However, I was writing a script which needed to be able to accept only certain parameters, which were mostly optional, and also take a list of filenames. I wanted this to be possible in any order, e.g.:

myscript --foo=foo --bar=bar file1 file2
myscript file1 file --foo foo

Getopt::Lucid makes this all pretty easy, and also makes the code pretty self-documenting, too. Straight from the documentation, showing the various types of parameters it can parse:

@specs = (
    Switch("version|V"),
    Counter("verbose|v"),
    Param("config|C"),
    List("lib|l|I"),
    Keypair("define"),
    Switch("help|h"),
);
 
$opt = Getopt::Lucid->getopt( \@specs );
 
$verbosity = $opt->get_verbose;
@libs = $opt->get_lib;
%defs = $opt->get_define;

A real-world example,from one of my scripts which handles ID3-tagging:

# The options we can take - these correspond to the name of the tag that
# they'll set:
my @options = qw(track title artist album comment);
 
my @option_specs = (
    Getopt::Lucid::Switch('verbose|v'),
    map { Getopt::Lucid::Param($_) } @options,
);
my $opt = Getopt::Lucid->getopt(\@option_specs);
 
my @tags_to_set = grep { $opt->{seen}{$_} } @options;
my @files = @{ $opt->{target} };
 
if (!@tags_to_set) {
    say "Nothing to do.  Use one or more of the options:\n" .
        join ', ', map {'--'.$_} @options;
    exit;
}

(The script then goes on to loop over all files, and use Music::Tag to set the ID3 tags requested).

I’ve been starting to make use of the new features introduced in perl 5.10 recently (after being constrained by my main dev environments still running perl 5.8.8, and not having the time to upgrade).

My favourite features so far are:

The smart match operator

The new smart-match operator, ~~, is a great example of DWIM.

A few examples:

if (@a ~~ 'foo')  # list contains at least one item equalling 'foo'
if (@a ~~ /foo/) # list contains at least one item matching /fo+/
if (@a ~~ @b)   # lists contain same values

That’s just a brief overview; there’s plenty more documentation

say

Not a big change, but the new say keyword acts just like print, but adds an implicit newline to the end – so say 'Hello'; is just the same as print "Hello\n";

It’s more useful in cases where you would have had to add parenthesis to get correct precedence – something like: print join(';', @foo) . "\n"; can now be written more concisely as just say join ';', @foo;.

Switch (given) statement

given ($foo) {
    when (/^abc/) { abc(); }
    when (/^def/) { def(); }
    when (/^xyz/) { xyz(); }
    default { die "Unrecognised foo"; }
}

Defined-or

// is now the defined-or operator.

It's pretty common to use conditional assignments like: $a ||= $b to assign to $a unless $a already has a value. Now you can use $a //= $b to test for definedness rather than truthiness.

Likewise, if ($hash{foo} // $hash{bar}) will be true if either of them is defined (even if they're defined but have a false value).

Named regex captures

Parenthesised sub-expressions in regular expressions can now be given a name, and accessed via the special %+ hash:

if ($foo =~ m{ (?<year> \d{4} ) - (?<month> \d{2}) - (?<day> \d{2}) }xms) {
    say "Year: $+{year}";
}

The features above are my own personal favourites, in no particular order. The full (large) set of changes can be found in the perldelta for 5.10.0.