recursive_type.cr

alias RecursiveType = String | 
                      Int32 |
                      Array(RecursiveType) |
                      Hash(String, RecursiveType)

extra_fields = {} of String => RecursiveType
extra_fields = {
  "teste" => "teste",
  "foo" => {
    "bar" => "blabla"
  },
  "array" => ["teste", "teste"],
  "deep" => [
    {
      "foo" => {
        "foo" => "bar"
      }
    }
  ]  
}