Enabling CORS for Cowboy REST API-Collection of common programming errors

How can I enable CORS for cowboy rest handler? I tried to add options/2 method, like this:

options(Req, State) ->
    {[
       {, },
       {, }
     ], Req, State}.

but this causes errors like:

Error in process  with exit value: {{case_clause,{[{,},{,}],{http_req,#Port,ranch_tcp,keepalive,,,{1,1},{{127,0,0,1},56522},,undefined,9090,,undefined,,undefined,,[],[{,},{,},{,},{,},{,},{,},{,},{,},{,}],[{,[]}],undefined,[],waiting,undefined,,false,waiting,[],,undefined},undefined...

Where is my mistake?

1. Cowboy documentation says you need to set header by using set_resp_headers, not return a list of headers:

    %% If you need to add additional headers to the response at this point,
    %% you should do it directly in the options/2 call using set_resp_headers.
   

   So your code should look like:

   options(Req, State) ->
       Req1 = cowboy_req:set_resp_header(, , Req),
       Req2 = cowboy_req:set_resp_header(, , Req1),
       {ok, Req2, State}.
   

   You can test with

   curl -H "Origin: http://example.com" \
     -H "Access-Control-Request-Method: GET" \
     -H "Access-Control-Request-Headers: X-Requested-With" \
     -X OPTIONS --verbose \
   http://localhost:8080
   * About to connect() to localhost port 8080 (#0)
   *   Trying 127.0.0.1... connected
   * Connected to localhost (127.0.0.1) port 8080 (#0)
   > OPTIONS / HTTP/1.1
   > User-Agent: curl/7.21.4 (universal-apple-darwin11.0) libcurl/7.21.4 OpenSSL/0.9.8r     zlib/1.2.5
   > Host: localhost:8080
   > Accept: */*
   > Origin: http://example.com
   > Access-Control-Request-Method: GET
   > Access-Control-Request-Headers: X-Requested-With
   >
   < HTTP/1.1 200 OK
   < connection: keep-alive
   < server: Cowboy
   < date: Mon, 25 Mar 2013 15:59:11 GMT
   < content-length: 0
   < access-control-allow-methods: GET, OPTIONS
   < access-control-allow-origin: *
   <
   * Connection #0 to host localhost left intact
   * Closing connection #0