Create your own Dancer 2 DSL

As the previous articles explained, Dancer 2 is a nice and shiny rewrite of Dancer 1, with a lot of added benefits and strengths. One of them is that it's properly object oriented, thanks to Moo.

Thanks to that redesign of the framework, Dancer's core elements are now objects, roles, or some kind of light factories. That is also true for Dancer DSL, which means that it can be extended or replaced altogether by one of your own!

But first, let's step back a bit.

DSL-what?

DSL means Domain-Specific Language (see wikipedia). It's basically what makes Dancer so easy to use. In our case it's a collection of keywords, which allows the developer to tell to Dancer what to do.

Let's look at a bit of code:

prefix '/debug';

get '/display_env' => sub {
    to_dumper(request->env);
};

hook before => sub {
    dancer_app->environment ne 'development'
      and halt;
}

The example above uses no less than 7 keywords! Dancer's DSL is everywhere in a Dancer-powered application. If you actually wonder what the DSL keywords are in this example, they are : prefix, get, to_dumper, request, hook, dancer_app and halt.

So the idea is to extend or change Dancer DSL per web application, without writing too much code. To be able to do that, let's look at what the default Dancer's DSL is.

DSL-how ?

How's the default DSL implemented? It very simply lies in the package Dancer::Core::DSL. You can look at the code here.

But, basically, it's a Moo class consuming the Dancer::Core::Role::DSL role, and defining a list of keywords and their implementations. Keywords are names, and the implementations are methods of the DSL class. The keywords can be of two kinds, the ones that can be called anywhere, and the ones that can be called only inside a route. The implementations make heavily usage of the other Dancer's concept, like the app, settings, sessions, settings, and so on.

Extend the default DSL

So, what we want to do here is introduce new keywords for some fun. As an example, we are going to translate some keywords in french. Namely:

  • get

    will also be availabe as prend

  • post

    will also be availabe as envoie

  • halt

    will also be availabe as stop

  • header

    will also be availabe as entete

  • cookie

    will also be availabe as gateau

Not very useful and not very christmas related, but it'll still do!

Create a new DSL

We can't just extend the default DSL, we need to create a new DSL module, which will itself extend the default one. So, from what we have said and from the source code of the core DSL, what we need is to create a new class FrenchDSL, that extends Dancer::Core::DSL:

package MyDancerDSL;
   
use Moo;
   
extends 'Dancer::Core::DSL';

Pretty easy, right? Now, let's see. We still want to benefit of all the default keywords, but we want to add our translated ones. The keywords are defined by the dsl_keyword method that returns them as an Array of Arrays. We can use the around method modifier of Moo to add our keywords at the end:

around dsl_keywords => sub {
    my $orig = shift;
    my $keywords = $orig->(@_);
    
    push @$keywords, 
      [ gateau => 0 ],
      [ moteur => 1 ],
      [ stop => 0 ],
      [ prend =>  1 ],
      [ envoie =>  1 ],
      [ entete => 0 ];
    
    return $keywords;
};

The 1's and 0's indicate if the keywords are global or can only be called from within route code.

Third step: let's implement our new keywords. That's very easy because we are just aliasing existing keywords:

sub gateau { goto &Dancer::Core::DSL::cookie };
sub moteur { goto &Dancer::Core::DSL::engine };
sub stop { goto &Dancer::Core::DSL::halt };
sub prend { goto &Dancer::Core::DSL::get };
sub envoie { goto &Dancer::Core::DSL::post };
sub entete { goto &Dancer::Core::DSL::header };

1;

Pretty easy. Adding a final true value and save the whole in FrenchDSL.pm in a place where it can be use'd.

use your new DSL

Now that we have a new Dancer DSL that extends the default one, how do we ask to Dancer to use this one instead of the default one ? By specifying it at use time, in our application script:

use strict;
use warnings;
    
use Dancer dsl => 'FrenchDSL';
    
envoie '/'  => sub {
    "That's the / route, you GET command!";
};
    
prend '/'  => sub {
    "A POST has been sent to / !";
};

dance;

We are coming to the end of the article. We hope that you have discovered that Dancer 2 allows easy and flexible extending, and that its guts are not that complicated. The best advice would be: use Dancer 2 in new web developments, and look at the code! it's not that big, and it's quite straightforward.

AUTHOR

Damien "dams" Krotkine

blog comments powered by Disqus