Question

If a Ruby method creates threads, but joins them within the method, is that still considered a side-effect from a functional programming perspective? The implementation I’m using is YARV Ruby, in which threads are created by the operating system, albeit with a GVL (Global VM Lock).

The reason the threads are being created is to do system calls, which is a side effect anyway, but I’ve never come across anyone saying whether threads are or aren’t a side effect.

Simplified pseudo-code for this would look somewhat like the following:

  def run_tasks
    input_files = (1..4).map {|i| "input_#{i}"}
    output_files = (1..4).map {|i| "output_#{i}"}
    threads = [input_files, output_files].transpose.map |input_file, output_file|
      # system itself is a side effect
      Thread.new { system(["sub_program.rb", input_file, output_file]) }
    end
    threads.map(&:join)
  end

Answer 1

The passage of time is seldom considered a side-effect, so the use of threads to cause less time to take place is not a side-effect. This is analogous to the use of memoization to improve performance, which is also not a side-effect, even though it modifies hidden state.

Note that input_files.zip(output_files) will work as well as [input_files, output_files].transpose, and may communicate your intent better.