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Catalyst::TraitFor::Request::ContentNegotiationHelpers - assistance with content negotiation


For Catalyst v5.90090+

    package MyApp;

    use Catalyst;


For Catalyst older than v5.90090

    package MyApp;

    use Catalyst;
    use CatalystX::RoleApplicator;


In a controller:

    package MyApp::Controller::Example;

    use Moose;
    use MooseX::MethodAttributes;

    sub myaction :Local {
      my ($self, $c) = @_;
      my $best_media_type = $c->req->choose_media_type('application/json', 'text/html');

    sub choose :Local {
      my ($self, $c) = @_;
      my $body = $c->req->on_best_media_type(
        'no_match' => sub { 'none' },
        'text/html' => sub { 'html' },
        'application/json' => sub { 'json' });


    sub filter : Local {
      my ($self, $c) = @_;
      my @acceptable = $c->req->accepts_media_type('image/jpeg', 'text/html', 'text/plain');


When using Catalyst and developing web APIs it can be desirable to examine the state of HTTP Headers of the request to make decisions about what to do, for example what format to return (HTML, XML, JSON, other). This role can be applied to your Catalyst::Request to add some useful helper methods for the more common types of server side content negotiation.

Most of the real work is done by HTTP::Headers::ActionPack::ContentNegotiation, this role just seeks to make it easier to use those features.


This role defines the following attributes.


This is a 'bare' attribute with no direct accessors. It expose a few methods to assist in content negotation.


This role defines the following methods:

choose_media_type (@array_of_types)

Given an array of possible media types ('application/json', 'text/html', etc.) return the one that is the best match for the current request (by looking at the current request ACCEPT header, parsing it and comparing).

  my $best_type = $c->req->accepts_media_type('image/jpeg', 'text/html', 'text/plain');

Returns undefined if no types match.

accepts_media_type ($type | @types)

For each media type passed as in a list of arguments, return that media type IF the type is acceptable to the requesting client. Can be used in boolean context to determine if a single or list of types is acceptable or as a filter to permit on those types that are acceptable:

    if($c->req->accepts_media_type('application/json')) {
      # handle JSON

    my @acceptable = $c->req->accepts_media_type('image/jpeg', 'text/html', 'text/plain');

If nothing is acceptable an undef will be returned.

on_best_media_type (%callbacks)

Given a hash where the keys are media types and the values are coderefs, execute and return the value of the coderef whose key is the best match for that media type (based on the result of "choose_media_type". For example:

    my $body = $c->req->on_best_media_type(
      'no_match' => sub { 'none' },
      'text/html' => sub { 'html' },
      'application/json' => sub { 'json' });


The coderef will receive the current request object as its single argument.

If there are no matches, execute the coderef associated with a 'no_match' key or return undef if no such key exists. When executing the 'no_match' callback (if any) we also pass a hash of the other callbacks, which you might use for setting a default response, or to inspect as part of the information required.

    'no_match' => sub {
      my ($req, %callbacks) = @_;
      my @allowed = keys %callbacks;

In this case the 'no_match' callback is removed from '%callbacks' passed to prevent the possibility of recursion.

choose_language (@array_of_langauges)

Given an array of possible media types ('en-US', 'es', etc.) return the one that is the best match for the current request.

accepts_language ($type)

Like "accepts_media_type" but for request language.

on_best_language (%callbacks)

Works like "on_best_media_type" but matches language.

choose_charset (@array_of_character_sets)

Given an array of possible media types ("UTF-8", "US-ASCII", etc.) return the one that is the best match for the current request.

accepts_charset ($type)

Like "accepts_media_type" but for request character set.

on_best_charset (%callbacks)

Works like "on_best_media_type" but matches charset.

choose_encoding (@array_of_encodings)

Given an array of possible encodings ("gzip", "identity", etc.) return the one that is the best match for the current request.

accepts_encoding ($type)

Like "accepts_media_type" but for request encoding.

on_best_encoding (%callbacks)

Works like "on_best_media_type" but matches encoding.





These are methods that map directly to the underlying HTTP::Headers::ActionPack::ContentNegotiation object. The are basicallty the same functionality except they require two arguments (an additional one to support the HTTP header string). You generally won't need them unless you need to compare am HTTP header that is not one that is part of the current request.

Action Roles

The following ActionRoles are part of this distribution and are offered as an alternative method for constructing your controllers



John Napiorkowski


Catalyst, Catalyst::Request, HTTP::Headers::ActionPack::ContentNegotiation


Copyright 2015, John Napiorkowski

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.