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";
    $app;
};

# use URLMap

builder {
    mount "/foo" => builder {
        enable "Plack::Middleware::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, 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 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 is supposed to return another code reference which is PSGI application that consumes $env in 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 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:

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

• builder { ... } should normally the last statement of a .psgi file, because the return value of builder is the application that actually is executed.

• You can nest your builder block, mixed with mount (see URLMap support above):

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

  will locate the $app under /foo/bar since the inner builder block puts it under /bar and it results a new PSGI application which is located under /foo because of the outer builder block.

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

cpanm Plack

perl -MCPAN -e shell
install Plack