The organic way a Perl script grows usually starts with a few statements, conditionals and loops. Sometimes this can grow to hundreds or even thousands of lines. Then parts of the script might slowly be turned into functions, but it is usually very painful.

Moo can help us, even when we write a single script, without modules, to write much cleaner code.

Of course these scripts will also need to accept parameters from the command line. Let's see how can this be done.

A single-file script using Moo

Let's save the following code as process.pl:

use Moo;
use 5.010;

sub run {
    my ($self) = @_;
    say 'Processing ...';
}

main->new->run;

We don't create a separate file with a separate module, but we use Moo right at the beginning of the script. (Of course we could have added a sh-bang line like this: #!/usr/bin/env perl as the first line of the script, but that is only needed if we would like to turn the script into executable on Linux/Unix.)

Every perl script already has an implicit package declaration, and the package of script is called main. Probably because programs written in C required a function called main. By adding a use Moo; statement at the top of our script we effectively turn our script into a class, with use strict; and use warnings; implicitly turned on.

We put our main code in a method, in our case called run, and then we create an instance of this main class by calling the new constructor. It returns an object and then we call the run method of that object.

We could have written it in two separate statements like this:

my $object = main->new;
$object->run;

but we don't really need the temporary $object variable.

When we run this script using perl process.pl The output will be Processing ...

Attributes and command line options

We will need variables in our script, but as we try to use the declarative style that comes with Moo, we declare an object-attribute in the main class. This is done exactly the same way we declared attributes in a regular Moo-based class: has file => (is => 'ro', required => 1);

The script will require a file to work on.

The name of the file needs to be passed to the constructor. It might be hard-coded in the script, in which case we would write: main->new(file => 'data.txt')->run;, but most command line scripts are supposed to get the parameters from the user passed in on the command line.

So instead of hard-coding the name of the file we expect it to be the parameter. We can the write:

my $file = shift or die "Usage: $0 FILE\n";

main->new(file => $file)->run;

This will get the first element from the @ARGV created from the command line, and pass it to the constructor.

The new version of the script looks like this:

use Moo;
use 5.010;

has file => (is => 'ro', required => 1);

sub run {
    my ($self) = @_;
    say 'Processing ' . $self->file;
}

my $file = shift or die "Usage: $0 FILE\n";

main->new(file => $file)->run;

If we run it without any parameters: perl process.pl we will get Usage: process.pl FILE and if we provide a parameter perl process.pl data.txt it will print Processing data.txt.

So far it looks ok, though there is some duplication in our code. Actually we had to mention file 3 times in our code. In the has statement, in the shift, and when passing it to the constructor.

Our code is not DRY.

We'll have to deal with that later, but let's look at another problem:

Add an optional parameter

In most scripts we will want more than one parameter. For example often we might want to be able to turn on some debugging or verbose mode to see what our script does. So let's add an optional attribute (it is not required) called verbose. We need to declare it: has verbose => (is => 'ro');, we need to receive it from the command line along with the other parameter: my ($file, $verbose) = @ARGV;, and finally we need to pass it to the constructor: main->new(file => $file, verbose => $verbose)->run;.

Again, we had to repeat ourself 3 times, but probably worse than that, the growing number of command line parameters require us to remember their order. It won't be easy to add another parameter after the optional one, and it is totally not obvious what the 1 means on the command line. (It means TRUE.)

The new version of the script looks like this:

use Moo;
use 5.010;

has verbose => (is => 'ro');
has file    => (is => 'ro', required => 1);

sub run {
    my ($self) = @_;
    if ($self->verbose) {
        say 'Processing ' . $self->file;
    }
}

my ($file, $verbose) = @ARGV;
die "Usage: $0 FILE [1]\n" if not $file;

main->new(file => $file, verbose => $verbose)->run;

perl process.pl will print Usage: process.pl FILE [1]

perl process.pl data.txt will be silent.

perl process.pl data.txt 1 will print Processing data.txt.

So will perl process.pl data.txt 42 by the way.

Two big problems

We have two major issues:

  1. Every new option has to be dealt with in 3 places.
  2. The positional nature of the options does not provide enough flexibility.

The latter could be dealt with Getopt::Long, but using Moo we have a better way to handle this.

The generally accepted solution to the problem of the positional arguments is to use named options. Each option will have a name preceded by a dash. Either a single-character name preceded by a single dash like this:

perl process.pl -f data.txt

or a long-name preceded by two dashes like this:

perl process.pl --file data.txt

Options that can be either true or false (as in the case of the $verbose value, can be set by supplying a name without any value: -v for the single-character version, or --verbose for the long version.

Command line options using MooX::Options

MooX::Options is an extension of Moo that solves problem 2 above. Let's see the new version of our code. Still not perfect, but a step in the right direction:

use Moo;
use MooX::Options;
use 5.010;

option verbose => (is => 'ro');
option file    => (is => 'ro', required => 1);

sub run {
    my ($self) = @_;
    if ($self->verbose) {
        say 'Processing ' . $self->file;
    }
}

main->new_with_options->run;

After loading the module with use MooX::Options;, we replaced the has statements by option statements. The option statements do everything the has statements do, but they also tell MooX::Options to accept these words as the names of command line options.

In the rest of the code we don't need to deal with @ARGV directly, and instead of calling new, we call the new_with_options method added by MooX::Options.

The result is, that if we run perl process.pl we get the following output:

file is missing
USAGE: process.pl [-h] [long options...]
	--file       no doc for file
	--verbose    no doc for verbose
	-h --help    show this help message

That looks interesting. Without doing anything further we already have a skeleton usage page including a --help flag!

If we follow the hint in the usage page we'll run perl process.pl --file data.txt that will print nothing. Which is expected of course. If we also add the --verbose flag and run perl process.pl --file data.txt --verbose we get a strange result: Processing 1. After referring to the documentation we realize that we still have to tell MooX::Options that the --file option expects a value after it, while the --verbose does not. Actually, the default is that it expects no value, and thus when we supplied the --file flag it set the file attribute to the value 1 (meaning TRUE).

Loading MooX::Options adds a few more keywords to the syntax of Moo. Specifically we can now add

format => 's'

to the declaration of file that means we are expecting a string value after the name.

The full declaration will then look like this:

option file    => (is => 'ro', required => 1, format => 's');

Running perl process.pl --file data.txt --verbose will now yield: Processing data.txt as expected.

Unfortunately the output of perl process.pl does not change, even though I think it could have indicated that --file expects a value. (Feature request submitted.)

In addition to s, the format option can also get the value i indicating an integer or f indicating a floating point number. (Thought it seems f also accepts values such as 1.2.3.4.5. Bug reported.)

Add documentation

As the help output indicates, there is a way to add documentation to the options. MooX::Options provides the doc keyword that allows us to supply a short description for each option:

option verbose => (is => 'ro', doc => 'Print details');
option file    => (is => 'ro', required => 1, format => 's',
    doc => 'File name to be processed');

Running perl process.pl --help will then print:

USAGE: process6.pl [-h] [long options...]
	--file       File name to be processed
	--verbose    Print details
	-h --help    show this help message

Much nicer, and all the information about the options are in a single place. Our code is DRY.

Attributes that are not options

In addition to the two attributes that can provided as command line arguments, our code might require other attributes that are not filled from the command line. All those attributes can be still declared using the has statement described earlier:

For example: has counter => (is => 'rw', default => 0); will add an attribute to count something but it will not appear in the help output, and if we supply it on the command line: perl process7.pl --file data.txt --verbose --counter we will get an Unknown option: counter error and usage explanation.

Multiple values

It is also possible to tell MooX::Options that our script will accept the same flag multiple times. For example we would like to get a list of IP addresses. We do that by adding a @ character to the format like this:

option ips => (is => 'ro', doc => 'IP addresses', format => 's@');

Using Data::Dumper we then print out the content of ips attribute:

use Data::Dumper qw(Dumper);
sub run {
    my ($self) = @_;
    if ($self->verbose) {
        say 'Processing ' . $self->file;
        say Dumper $self->ips;
    }
}

Let's see what is the output:

$ perl process7.pl --file data.txt --verbose
Processing data.txt
$VAR1 = undef;

$ perl process7.pl --file data.txt --verbose --ip 1.1.1.1
Processing data.txt
$VAR1 = [
          '1.1.1.1'
        ];

$ perl process7.pl --file data.txt --verbose --ip 1.1.1.1 --ip 2.2.2.2
Processing data.txt
$VAR1 = [
          '1.1.1.1',
          '2.2.2.2'
        ];

The nice part is that we can actually supply a name shorter than the full name of the attribute: --ip instead of --ips which looks much more intuitive in our case.

When we supply --ip one or more times, we get an ARRAY ref with the list of values. On the other hand, when there are no values, the attribute ips will remain undef.

This means when we need to write code to processes all the IPs we'll have to write something like this:

if ($self->ips) {
    foreach my $ip (@{ $self->ips }) {
        say $ip;
    }
}

We have to protect the loop by an if-statement so we don't get a Can't use an undefined value as an ARRAY reference at ... error.

In some cases the above if-statement is necessary anyway, but sometimes we really just want to iterate over all the elements. In that case it is probably better to set ithe default value of the ips attribute to be an empty array. (Remember, we have to use an anonymous function to do this.)

option ips => (is => 'ro', doc => 'IP addresses', format => 's@',
    default => sub { [] } );

Then the code handling the IP addresses will be simplified to the loop:

foreach my $ip (@{ $self->ips }) {
    say $ip;
}

The full example

Let's conclude this article with the full example we have developed:

use Moo;
use MooX::Options;
use 5.010;
use Data::Dumper qw(Dumper);

option verbose => (is => 'ro', doc => 'Print details');
option file    => (is => 'ro', required => 1, format => 's',
    doc => 'File name to be processed');

has counter => (is => 'rw', default => 0);
option ips => (is => 'ro', doc => 'IP addresses', format => 's@', default => sub { [] } );

sub run {
    my ($self) = @_;
    if ($self->verbose) {
        say 'Processing ' . $self->file;
        say Dumper $self->ips;
        foreach my $ip (@{ $self->ips }) {
            say $ip;
        }
    }
}

main->new_with_options->run;

Exercise 1

Allow the user to supply the list of IP addresses as a comma separated list with a single --ips.

perl process.pl --file data.txt --verbose --ips 1.1.1.1,2.2.2.2

Please note, it seems due to a bug we need to use the full name of the attribute here --ips as --ip won't work. (The bug was reported against v3.83 of MooX::Options.)

Exercise 2

Allow the use of the short name for the verbose option as -v

perl process8.pl --file data.txt -v --ips 1.1.1.1,2.2.2.2

The source code of the most recent version of this script can be found in GitHub. You are more than welcome to fork that repository, send your solution as a pull request and comment on other peoples solution.