Provided by: libdancer-perl_1.3521+dfsg-1_all 
NAME
Dancer::Introduction - A gentle introduction to Dancer
VERSION
version 1.3521
DESCRIPTION
Dancer is a free and open source micro web application framework written in Perl.
INSTALL
Installation of Dancer is simple:
perl -MCPAN -e 'install Dancer'
Thanks to the magic of cpanminus, if you do not have CPAN.pm configured, or just want a quickfire way to
get running, the following should work, at least on Unix-like systems:
wget -O - http://cpanmin.us | sudo perl - Dancer
(If you don't have root access, omit the 'sudo', and cpanminus will install Dancer and prereqs into
"~/perl5".)
SETUP
Create a web application using the dancer script:
dancer -a MyApp
Run the web application:
cd MyApp
bin/app.pl
You can read the output of "bin/app.pl --help" to change any settings such as the port number.
View the web application at:
http://localhost:3000
USAGE
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');
};
HTTP METHODS
Here are some of the standard HTTP methods which 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.
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
};
ROUTE HANDLERS
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.
NAMED MATCHING
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!");
};
WILDCARDS MATCHING
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
};
REGULAR EXPRESSION MATCHING
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 $name";
};
CONDITIONAL MATCHING
Routes may include some matching conditions (on the useragent and the hostname at the moment):
get '/foo', {agent => 'Songbird (\d\.\d)[\d\/]*?'} => sub {
'foo method for songbird'
}
get '/foo' => sub {
'all browsers except songbird'
}
PREFIX
A prefix can be defined for each route handler, like this:
prefix '/home';
From here, any route handler is defined to /home/*
get '/page1' => sub {}; # will match '/home/page1'
You can unset the prefix value
prefix '/'; # or: prefix undef;
get '/page1' => sub {}; # will match '/page1'
Alternatively, to prevent you from ever forgetting to undef the prefix, you can use lexical prefix like
this:
prefix '/home' => sub {
get '/page1' => sub {}; # will match '/home/page1'
}; ## prefix reset to previous value on exit
get '/page1' => sub {}; # will match '/page1'
ACTION SKIPPING
An action can choose not to serve the current request and ask Dancer to process the request with the next
matching route.
This is done with the pass keyword, like in the following example
get '/say/:word' => sub {
return pass if (params->{word} =~ /^\d+$/);
"I say a word: ".params->{word};
};
get '/say/:number' => sub {
"I say a number: ".params->{number};
};
DEFAULT ERROR PAGES
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.
EXECUTION ERRORS
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.
HOOKS
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';
request->path_info('/foo/oversee')
};
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;
request->path_info('/login');
}
};
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.
CONFIGURATION AND ENVIRONMENTS
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
load
You can use the load method to include additional routes into your application:
get '/go/:value', sub {
# foo
};
load 'more_routes.pl';
# then, in the file more_routes.pl:
get '/yes', sub {
'orly?';
};
load is just a wrapper for require, but you can also specify a list of routes files:
load 'login_routes.pl', 'session_routes.pl', 'misc_routes.pl';
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/Routes.pm:
use Dancer ':syntax';
get '/user/view/:id' => sub {
...
};
LOGGING
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";
USING TEMPLATES
VIEWS
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/hello.tt view can contain the following code:
<html>
<head></head>
<body>
<h1>Hello <% params.name %></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 %>
</body>
</html>
LAYOUTS
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/main.tt:
<html>
<head><% page_title %></head>
<body>
<div id="header">
...
</div>
<div id="content">
<% content %>
</div>
</body>
</html>
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
STATIC DIRECTORY
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 example.com/css/style.css.
STATIC FILE FROM A ROUTE HANDLER
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'
};
SETTINGS
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.
SERIALIZERS
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:
{"number":100234,"foo":42,"list":["one","two","three"]}
The following serializers are available, be aware they dynamically depend on Perl modules you may not
have on your system.
JSON
requires JSON
YAML
requires YAML
XML requires XML::Simple
Mutable
will try to find the appropriate serializer using the Content-Type and Accept-type header of the
request.
EXAMPLE
This is a possible webapp created with Dancer:
#!/usr/bin/perl
# 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->dance;
AUTHOR
Dancer Core Developers
COPYRIGHT AND LICENSE
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.
perl v5.36.0 2023-02-10 Dancer::Introduction(3pm)