class Logger
  class Formatter
    public

    attr_accessor datetime_format: String?

    def call: (String severity, Time time, untyped progname, untyped msg) -> String

    private

    def format_datetime: (Time time) -> untyped

    def initialize: () -> void

    def msg2str: (String | Exception | untyped msg) -> String
  end

  interface _Formatter
    def call: (String severity, Time time, untyped progname, untyped msg) -> _ToS
  end
end

Logger::Formatter::Format: String