Tatsuhiko Miyagawa > Plack-1.0005 > Plack::Builder

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NAME ^

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

SYNOPSIS ^

  # in .psgi
  use Plack::Builder;

  my $app = sub { ... };

  builder {
      enable "Deflater";
      enable "Session", store => "File";
      enable "Debug", panels => [ qw(DBITrace Memory Timer) ];
      enable "+My::Plack::Middleware";
      $app;
  };

  # use URLMap

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

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

  # using OO interface

  my $builder = Plack::Builder->new();
  $builder->add_middleware('Foo', opt => 1);
  $app = $builder->mount('/app' => $app);
  $app = $builder->to_app($app);

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 enable 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. "Plack::Middleware::" is added as a prefix by default. So:

  builder {
      enable "Foo";
      enable "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 enable middleware from outer to inner.

INLINE MIDDLEWARE ^

Plack::Builder allows you to code middleware inline using a nested code reference.

If the first argument to enable is a code reference, it will be passed an $app and should return another code reference which is a PSGI application that consumes $env at runtime. So:

  builder {
      enable sub {
          my $app = shift;
          sub {
              my $env = shift;
              # do preprocessing
              my $res = $app->($env);
              # do postprocessing
              return $res;
          };
      };
      $app;
  };

is equal to:

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

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

URLMap support ^

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

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

  my $app = sub {
      my $env = shift;
      ...
  };

  builder {
      mount "/foo" => sub { ... };
      $app; # 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.

  builder {
      mount "/foo" => sub { ... };
      mount "/" => $app;
  }

Note that the builder DSL returns a whole new PSGI application, which means

CONDITIONAL MIDDLEWARE SUPPORT ^

You can use enable_if to conditionally enable middleware based on the runtime environment. See Plack::Middleware::Conditional for details.

SEE ALSO ^

Plack::Middleware Plack::App::URLMap Plack::Middleware::Conditional

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