Look on my works, ye Mighty, and despair!

Working with json in Erlang has vastly improved since the last time I looked at it, particularly with the addition of maps to the language (in R17).

-module(foo_handler).

-export([init/3]).
-export([allowed_methods/2, content_types_accepted/2]).
-export([foo/2]).

init(_Transport, _Req, []) ->
    {upgrade, protocol, cowboy_rest}.

allowed_methods(Req, State) ->
    {[<<"POST">>], Req, State}.

content_types_accepted(Req, State) ->
    {[
        {<<"application/json">>, foo}
    ], Req, State}.

foo(Req, State) ->
    {ok, Body, Req1} = cowboy_req:body(Req),
    Json = jiffy:decode(Body, [return_maps]),
    Bar = maps:get(<<"bar">>, Json),
    {ok, Res} = do_something(Bar),
    Body = jiffy:encode(#{result => Res}),
    {true, cowboy_req:set_resp_body(Body, Req1), State}.