module Concurrent::Options

def self.executor(executor_identifier)
  case executor_identifier
  when :fast
    Concurrent.global_fast_executor
  when :io
    Concurrent.global_io_executor
  when :immediate
    Concurrent.global_immediate_executor
  when Concurrent::ExecutorService
    executor_identifier
  else
    raise ArgumentError, "executor not recognized by '#{executor_identifier}'"
  end
end