gistfile1.rb

require "rubygems"
require "sinatra"
require "json"

class Jsonify
  def initialize(app)
    @app = app
  end
  
  def call(env)
    status, headers, response = @app.call(env)
    response = response.to_json
    headers["Content-Length"] = response.length.to_s
    headers["Content-Type"] = "application/json"
    [status, headers, response]
  end
end

use Jsonify

get "/foo" do
  { "foo" => "bar", "baz" => [ "blat", "honk", "hork" ] }
end