Dancer::Introduction(3) A gentle introduction to Dancer


version 1.3202


Dancer is a free and open source micro web application framework written in Perl.


Installation of Dancer is simple:

    perl -MCPAN -e 'install Dancer'

Thanks to the magic of cpanminus, if you do not have configured, or just want a quickfire way to get running, the following should work, at least on Unix-like systems:

    wget -O - | sudo perl - Dancer

(If you don't have root access, omit the 'sudo', and cpanminus will install Dancer and prereqs into "~/perl5".)


Create a web application using the dancer script:

    dancer -a MyApp

Run the web application:

    cd MyApp

You can read the output of "bin/ --help" to change any settings such as the port number.

View the web application at:



When Dancer is imported to a script, that script becomes a webapp, and at this point, all the script has to do is declare a list of routes. A route handler is composed by an HTTP method, a path pattern and a code block. "strict" and "warnings" pragmas are also imported with Dancer.

The code block given to the route handler has to return a string which will be used as the content to render to the client.

Routes are defined for a given HTTP method. For each method supported, a keyword is exported by the module.

The following is an example of a route definition. The route is defined for the method 'get', so only GET requests will be honoured by that route:

    get '/hello/:name' => sub {
        # do something
        return "Hello ".param('name');


Here are some of the standard HTTP methods which you can use to define your route handlers.
The GET method retrieves information (when defining a route handler for the GET method, Dancer automatically defines a route handler for the HEAD method, in order to honour HEAD requests for each of your GET route handlers). To define a GET action, use the get keyword.
The POST method is used to create a resource on the server. To define a POST action, use the post keyword.
The PUT method is used to update an existing resource. To define a PUT action, use the put keyword.
The DELETE method requests that the origin server delete the resource identified by the Request-URI. To define a DELETE action, use the del keyword.

To define a route for multiple methods you can also use the special keyword any. This example illustrates how to define a route for both GET and POST methods:

    any ['get', 'post'] => '/myaction' => sub {
        # code

Or even, a route handler that would match any HTTP methods:

    any '/myaction' => sub {
        # code


The route action is the code reference declared. It can access parameters through the `params' keyword, which returns a hashref. This hashref is a merge of the route pattern matches and the request params.

You can have more details about how params are built and how to access them in the Dancer::Request documentation.


A route pattern can contain one or more tokens (a word prefixed with ':'). Each token found in a route pattern is used as a named-pattern match. Any match will be set in the params hashref.

    get '/hello/:name' => sub {
        "Hey ".param('name').", welcome here!";

Tokens can be optional, for example:

    get '/hello/:name?' => sub {
        "Hello there " . (param('name') || "whoever you are!");


A route can contain a wildcard (represented by a '*'). Each wildcard match will be returned in an arrayref, accessible via the `splat' keyword.

    get '/download/*.*' => sub {
        my ($file, $ext) = splat;
        # do something with $file.$ext here


A route can be defined with a Perl regular expression.

In order to tell Dancer to consider the route as a real regexp, the route must be defined explicitly with "qr{}", like the following:

    get qr{/hello/([\w]+)} => sub {
        my ($name) = splat;
        return "Hello $name";


Routes may include some matching conditions (on the useragent and the hostname at the moment):

    get '/foo', {agent => 'Songbird (\d\.\d)[\d\/]*?'} => sub {
      'foo method for songbird'
    get '/foo' => sub {
      'all browsers except songbird'


A prefix can be defined for each route handler, like this:

    prefix '/home';

From here, any route handler is defined to /home/*

    get '/page1' => sub {}; # will match '/home/page1'

You can unset the prefix value

    prefix '/'; # or: prefix undef;
    get '/page1' => sub {}; # will match '/page1'

Alternatively, to prevent you from ever forgetting to undef the prefix, you can use lexical prefix like this:

    prefix '/home' => sub {
      get '/page1' => sub {}; # will match '/home/page1'
    }; ## prefix reset to previous value on exit
    get '/page1' => sub {}; # will match '/page1'


An action can choose not to serve the current request and ask Dancer to process the request with the next matching route.

This is done with the pass keyword, like in the following example

    get '/say/:word' => sub {
        return pass if (params->{word} =~ /^\d+$/);
        "I say a word: ".params->{word};
    get '/say/:number' => sub {
        "I say a number: ".params->{number};


When an error is rendered (the action responded with a status code different than 200), Dancer first looks in the public directory for an HTML file matching the error code (eg: 500.html or 404.html).

If such a file exists, it's used to render the error, otherwise, a default error page will be rendered on the fly.


When an error occurs during the route execution, Dancer will render an error page with the HTTP status code 500.

It's possible either to display the content of the error message or to hide it with a generic error page.

This is a choice left to the end-user and can be set with the show_errors setting.

Note that you can also choose to consider all warnings in your route handlers as errors when the setting warnings is set to 1.


Before hooks

Before hooks are evaluated before each request within the context of the request and can modify the request and response. It's possible to define variables which will be accessible in the action blocks with the keyword 'var'.

    hook 'before' => sub {
        var note => 'Hi there';
    get '/foo/*' => sub {
        my ($match) = splat; # 'oversee';
        vars->{note}; # 'Hi there'

For another example, this can be used along with session support to easily give non-logged-in users a login page:

    hook 'before' => sub {
        if (!session('user') && request->path_info !~ m{^/login}) {
            # Pass the original path requested along to the handler:
            var requested_path => request->path_info;

The request keyword returns the current Dancer::Request object representing the incoming request. See the documentation of the Dancer::Request module for details.

After hooks

"after" hooks are evaluated after the response has been built by a route handler, and can alter the response itself, just before it's sent to the client.

The hook is given the response object as its first argument:

    hook 'after' => sub {
        my $response = shift;
        $response->{content} = 'after hook got here!';

Before template hook

"before_template_render" hooks are called whenever a template is going to be processed, they are passed the tokens hash which they can alter.

    hook 'before_template_render' => sub {
        my $tokens = shift;
        $tokens->{foo} = 'bar';

The tokens hash will then be passed to the template with all the modifications performed by the hook. This is a good way to setup some global vars you like to have in all your templates, like the name of the user logged in or a section name.


Configuring a Dancer application can be done in many ways. The easiest one (and maybe the dirtiest) is to put all your settings statements at the top of your script, before calling the dance() method.

Other ways are possible, you can write all your setting calls in the file `appdir/config.yml'. For this, you must have installed the YAML module, and of course, write the conffile in YAML.

That's better than the first option, but it's still not perfect as you can't switch easily from an environment to another without rewriting the config.yml file.

The better way is to have one config.yml file with default global settings, like the following:

    # appdir/config.yml
    logger: 'file'
    layout: 'main'

And then write as many environment files as you like in appdir/environments. That way, the appropriate environment config file will be loaded according to the running environment (if none is specified, it will be 'development').

Note that you can change the running environment using the --environment command line switch.

Typically, you'll want to set the following values in a development config file:

    # appdir/environments/development.yml
    log: 'debug'
    startup_info: 1
    show_errors:  1

And in a production one:

    # appdir/environments/production.yml
    log: 'warning'
    startup_info: 0
    show_errors:  0


You can use the load method to include additional routes into your application:

    get '/go/:value', sub {
        # foo
    load '';
    # then, in the file
    get '/yes', sub {

load is just a wrapper for require, but you can also specify a list of routes files:

    load '', '', '';

Accessing configuration data

A Dancer application can access the information from its config file easily with the config keyword:

    get '/appname' => sub {
        return "This is " . config->{appname};

Importing just the syntax

If you want to use more complex file hierarchies, you can import just the syntax of Dancer.

    package App;
    use Dancer;            # App may contain generic routes
    use App::User::Routes; # user-related routes

Then in App/User/

    use Dancer ':syntax';
    get '/user/view/:id' => sub {


It's possible to log messages sent by the application. In the current version, only one method is possible for logging messages but future releases may add additional logging methods, for instance logging to syslog.

In order to enable the logging system for your application, you first have to start the logger engine in your config.yml

    logger: 'file'

Then you can choose which kind of messages you want to actually log:

    log: 'debug'     # will log debug, warning, error and info messages
    log: 'info'      # will log info, warning and error messages
    log: 'warning'   # will log warning and error messages
    log: 'error'     # will log error messages

A directory appdir/logs will be created and will host one logfile per environment. The log message contains the time it was written, the PID of the current process, the message and the caller information (file and line).

To log messages, use the debug, info, warning and error functions. For instance:

    debug "This is a debug message";



It's possible to render the action's content with a template; this is called a view. The `appdir/views' directory is the place where views are located.

You can change this location by changing the setting 'views', for instance if your templates are located in the 'templates' directory, do the following:

    set views => path(dirname(__FILE__), 'templates');

By default, the internal template engine is used (Dancer::Template::Simple) but you may want to upgrade to Template::Toolkit. If you do so, you have to enable this engine in your settings as explained in Dancer::Template::TemplateToolkit. If you do so, you'll also have to import the Template module in your application code. Note that Dancer configures the Template::Toolkit engine to use <% %> brackets instead of its default [% %] brackets, although you can change this in your config file.

All views must have a '.tt' extension. This may change in the future.

In order to render a view, just call the 'template' keyword at the end of the action by giving the view name and the HASHREF of tokens to interpolate in the view (note that the request, session and route params are automatically accessible in the view, named request, session and params):

    use Dancer;
    use Template;
    get '/hello/:name' => sub {
        template 'hello' => { number => 42 };

And the appdir/views/ view can contain the following code:

        <h1>Hello <% %></h1>
        <p>Your lucky number is <% number %></p>
        <p>You are using <% request.user_agent %></p>
        <% IF session.user %>
            <p>You're logged in as <% session.user %></p>
        <% END %>


A layout is a special view, located in the 'layouts' directory (inside the views directory) which must have a token named `content'. That token marks the place where to render the action view. This lets you define a global layout for your actions. Any tokens that you defined when you called the 'template' keyword are available in the layouts, as well as the standard session, request, and params tokens. This allows you to insert per-page content into the HTML boilerplate, such as page titles, current-page tags for navigation, etc.

Here is an example of a layout: views/layouts/

        <head><% page_title %></head>
        <div id="header">
        <div id="content">
        <% content %>

This layout can be used like the following:

    use Dancer;
    set layout => 'main';
    get '/' => sub {
        template 'index' => { page_title => "Your website Homepage" };

Of course, if a layout is set, it can also be disabled for a specific action, like the following:

    use Dancer;
    set layout => 'main';
    get '/nolayout' => sub {
        template 'some_ajax_view',
            { tokens_var => "42" },
            { layout => 0 };



Static files are served from the ./public directory. You can specify a different location by setting the 'public' option:

    set public => path(dirname(__FILE__), 'static');

Note that the public directory name is not included in the URL. A file ./public/css/style.css is made available as


It's possible for a route handler to send a static file, as follows:

    get '/download/*' => sub {
        my $params = shift;
        my ($file) = @{ $params->{splat} };
        send_file $file;

Or even if you want your index page to be a plain old index.html file, just do:

    get '/' => sub {
        send_file '/index.html'


It's possible to change quite every parameter of the application via the settings mechanism.

A setting is key/value pair assigned by the keyword set:

    set setting_name => 'setting_value';

More usefully, settings can be defined in a YAML configuration file. Environment-specific settings can also be defined in environment-specific files (for instance, you might want extra logging in development). See the cookbook for examples.

See Dancer::Config for complete details about supported settings.


When writing a webservice, data serialization/deserialization is a common issue to deal with. Dancer can automatically handle that for you, via a serializer.

When setting up a serializer, a new behaviour is authorized for any route handler you define: any response that is a reference will be rendered as a serialized string, via the current serializer.

Here is an example of a route handler that will return a HashRef

    use Dancer;
    set serializer => 'JSON';
    get '/user/:id/' => sub {
        { foo => 42,
          number => 100234,
          list => [qw(one two three)],

As soon as the content is a reference - and a serializer is set, which is not the case by default - Dancer renders the response via the current serializer.

Hence, with the JSON serializer set, the route handler above would result in a content like the following:


The following serializers are available, be aware they dynamically depend on Perl modules you may not have on your system.

requires JSON
requires YAML
requires XML::Simple
will try to find the appropriate serializer using the Content-Type and Accept-type header of the request.


This is a possible webapp created with Dancer:

    # make this script a webapp
    use Dancer;
    # declare routes/actions
    get '/' => sub {
        "Hello World";
    get '/hello/:name' => sub {
        "Hello ".param('name');
    # run the webserver


Dancer Core Developers


This software is copyright (c) 2010 by Alexis Sukrieh.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.