Dancer::Introduction - A gentle introduction to Dancer


Installation of Dancer is simple:

    # perl -MCPAN -e 'install Dancer'


Create a web application using the dancer script:

    dancer -a MyApp


As soon as Dancer is imported to a script, that script becomes a webapp. All the script has to do is to declare a list of routes. A route handler is composed by an HTTP method, a path pattern and a code block. strict and warnings pragma 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.

Here is an example of a route definition:

    get '/hello/:name' => sub {
        # do something important here

        return "Hello ".params->{name};

The route is defined for the method 'get', so only GET requests will be honoured by that route.


All existing HTTP methods are defined in the RFC 2616

Here are the ones you can use to define your route handlers.

GET 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.
POST The POST method is used to create a resource on the server. To define a POST action, use the post keyword.
PUT The PUT method is used to update an existing resource. To define a PUT action, use the put keyword.
DELETE 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.

You can also use the special keyword any to define a route for multiple methods at once. For instance, you may want to define a route for both GET and POST methods, this is done like the following:

    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 ".params->{name}.", welcome here!";


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


Once the script is ready, you can run the webserver just by running the script. The following options are supported:

--port=XXXX set the port to listen to (default is 3000)
--daemon run the webserver in the background
--help display a detailed help message


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 {
        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 filters

Before filters 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'.

    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:

    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 filters

after filters 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 filter is given the response object as its first argument:

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

Before template filters

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

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

The tokens hash will then be passed to the template with all the modifications performed by the filter. 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 commandline switch.

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

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

And in a production one:

    # appdir/environments/production.yml
    log: 'warning'
    access_log: 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 and errors
    log: 'warning'   # will log warning and errors
    log: 'error'     # will log only errors

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, warning and error methods, 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;
    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;
    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


By default, Dancer will automatically detect the mime-types to use for the static files accessed.

It's possible to choose specific mime-type per file extensions. For instance, we can imagine you want to serve *.foo as a text/foo content, instead of text/plain (which would be the content type detected by Dancer if *.foo are text files).

        mime_type foo => 'text/foo';

This configures the 'text/foo' content type for any file matching '*.foo'.


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'


Dancer automatically supports default caching for routes. What this means is that Dancer remembers for each path what route it took, so it doesn't have to match it again.

This makes things much faster, especially when dealing with many routes. There are default limitations on the size of the cache and the number of entries, so it doesn't get out of proportion.

Route caching can turned on using the route_cache option in the configuration:

    route_cache = 1

The default limitations are 10M in size or 600 entries in the cache, however you can override these by settings the following settings:

    # limiting the size of the route cache
    route_cache_size_limit: 50M

    # limiting the number of paths that will be cached
    route_cache_path_limit: 300


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 don't want auto_reload in production, and 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 non-scalar response 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 not a scalar - 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 ".params->{name}"

    # run the webserver