NAME

Plack::Builder - OO and DSL to enable Plack Middlewares

SYNOPSIS

  # in .psgi
  use Plack::Builder;

  my $app = sub { ... };

  builder {
      enable "Plack::Middleware::Foo";
      enable "Plack::Middleware::Bar", opt => "val";
      enable "Plack::Middleware::Baz";
      enable sub {
          my $app = shift;
          sub {
              my $env = shift;
              $app->($env);
          };
      };
      $app;
  };

  # use URLMap

  builder {
      mount "/foo" => builder {
          enable "Plack::Middleware::Foo";
          $app;
      };

      mount "/bar" => $app2;
      mount "http://example.com/" => builder { $app3 };
  };

DESCRIPTION

Plack::Builder gives you a quick domain specific language (DSL) to wrap your application with Plack::Middleware subclasses. The middleware you're trying to use should use Plack::Middleware as a base class to use this DSL, inspired by Rack::Builder.

Whenever you call add on any middleware, the middleware app is pushed to the stack inside the builder, and then reversed when it actually creates a wrapped application handler, so:

  builder {
      enable "Plack::Middleware::Foo";
      enable "Plack::Middleware::Bar", opt => "val";
      $app;
  };

is syntactically equal to:

  $app = Plack::Middleware::Bar->wrap($app, opt => "val");
  $app = Plack::Middleware::Foo->wrap($app);

In other words, you're supposed to add middleware from outer to inner.

Additionally, you can call enable with a coderef, which would take $app and returns a another psgi-app which consumes $env in runtime. So:

  my $mw = sub {
      my $app = shift;
      sub { my $env = shift; $app->($env) };
  };

  builder {
      enable $mw;
      $app;
  };

is syntactically equal to:

  $app = $mw->($app);

URLMap support

Plack::Builder has a native support for Plack::App::URLMap with mount method.

  use Plack::Builder;
  my $app = builder {
      mount "/foo" => $app1;
      mount "/bar" => builder {
          enable "Plack::Middleware::Foo";
          $app2;
      };
  };

See Plack::App::URLMap's map method to see what they mean. With builder you can't use map as a DSL, for the obvious reason :)

Note: Once you use mount in your builder code, you have to use mount for all the paths, including the root path (/). You can't have the default app in the last line of builder like:

  builder {
      mount "/foo" => sub { ... };
      sub {
          my $env = shift; # THIS DOESN'T WORK
      };
  };

You'll get warnings saying that your mount configuration will be ignored. Instead you should use mount "/" => ... in the last line to set the default fallback app.

SEE ALSO

Plack::Middleware Plack::App::URLMap