Customizing and extending your Dancer2 application generation

Dancer2 provides a useful command line that helps you generate a Dancer2 application skeleton without having to write it yourself.

For example, creating an app called My::Web::App, you can run the following:

dancer2 gen -a My::Web::App

The dancer2 command line has a few more options, which you can see, if you run dancer2 gen --help.

Changing the skeleton

Dancer2 generates a skeleton that it useful for most developers, but if you are a seasoned Dancer2 developer, you might have a set of preferences not represented in the default skeleton.

If it's different file setup that you want to have, you could partially achieve it with the dancer2 gen -s DIRECTORY, indicating a different skeleton directory. But that doesn't fix all of it.

To have full control over the entire scaffolding operation, you will need to have control of the command line implementation. Let me show you how.

Extending in a class

To extend the application in a class, you will need to write a new class with a new command. That class will then need to be loaded in your environment for you to enjoy it.

There are two options:

  • Write a new distribution with your new module and install it locally. Done.

  • Write a new module and make the directory in which it sits available in your $PERL5LIB environment variable.

    This option is more useful for companies that have a big library directory that is always available in the include directories list.

    An example of this:

    # Creating the directory
    $ mkdir -p /opt/perl/lib/Dancer2/CLI/Command/
    $ cd /opt/perl/lib/Dancer2/CLI/Command/
    # Putting a mostly-empty file
    $ echo -e "package Dancer2::CLI::Command::new;\n1;" >>
    # Now making sure this path is in PERL5LIB
    # (replace the bashrc file path with your system's path)
    echo "export PERL5LIB="/opt/perl/lib/:$PERL5LIB" >> /etc/bash.bashrc

Writing your own command

Dancer2 uses App::Cmd to implement the dancer2 command line utility. This means you can introduce additional commands by just implementing a class.

Writing a new command

package Dancer2::CLI::Command::activate
use strict;
use warnings;
use Path::Tiny qw< path >;
use App::Cmd::Setup -command;

sub description { 'Activating our application' }

sub opt_desc {
    return (
        [ 'directory|d', 'Application directory' ],
        # More options...

sub validate_args {
    my ( $self, $opt, $args ) = @_;
        or $self->usage_error('You did not provide a directory');

    path( $opt->{'directory'} )->is_dir
        or $self->usage_error('Path provided is not a directory');

sub execute {
    my ( $self, $opt, $args ) = @_;
    my $dir = $opts->{'directory'};
    # Implement the application activation
    # (Whatever that means...)


In this example, we introduce a new command to dancer2. As long as this class is available in your path (such as via your $PERL5LIB environment variable), you will be able to run the following:

$ dancer2 activate --directory foo/

(The implementation of what "activation" means in this context is left to the reader.)

But what if you want to provide an alteration of an existing command - the generation of the Dancer2 application?

Writing a new command

Let's say you have a set of adjustments you keep doing to your [company's] Dancer2 applications and you want to make these a default.

You can write it as a new command or you can subclass the existing command and do whatever alterations you want before, during, and after the generation of the skeleton.

package Dancer2::CLI::Command::mygen;
use strict;
use warnings;
use Cwd (); # Our own dependencies

# Subclass the existing "gen" command
use parent 'Dancer2::CLI::Command::gen';

sub execute {
  my ( $self, $opt, $args ) = @_;

  # Do whatever you want in this area, before we generate

  # For example, let's make sure the application
  # matches a certain naming convention

  my $app_name = $opt->{'application'};
  $app_name =~ /^My::Company::App::/
    or $self->usage_error('App must be prefixed by "My::Company::App");

  # Maybe check we are only scaffolding in a particular directory
  cwd() eq '/opt/my_company/webapps/'
      or $self->usage_error('Only create apps in our webapps directory');

  # At this point, we can run the original scaffolding
  $self->SUPER::execute( $opt, $args );

  # Now we finished generating, but we can contineu customizing what we have


Writing your own generation on top of the existing generation allows you to manage the input (including additional validation) and the output, giving you full control over the scaffolding process.

Some examples on which customizations you might want to perform:

  • Add additional default imported classes
  • Change the output directory name
  • Update a database that we have a new application
  • Update your team with an email or IRC/Slack message
  • Remove files that are not applicable for your setup and add new ones
  • Write helpful output for the developer who scaffolded the app


I hope you find these techniques useful to introduce customization for your home-grown Dancer2 application setup. I know I do. :)


This article has been written by Sawyer X for the Perl Dancer Advent Calendar 2018.


No copyright retained. Enjoy.

2018 // Sawyer X <>