Workaround for https://github.com/ruby-concurrency/concurrent-ruby/issues/611

workaround_lazy_chained_futures.rb

# Workaround for https://github.com/ruby-concurrency/concurrent-ruby/issues/611
#
# This approach works to *lazily* compose a chained future of
# sequential steps, where each step is a *zipped* future of a
# set of futures to execute in parallel at that step.
#
# Works with:
#
#     gem install concurrent-ruby-edge -v 0.2.3.pre3
#
# @author: "Marc Siegel" <[email protected]>
#
require "concurrent-edge"

@m = Mutex.new

def sync_puts(*args)
  @m.synchronize { puts(*args) }
end  

procs_to_delay_steps =
  Array.new(4) do |x|
    -> do
      sync_puts "\n-- starting step #{x}"

      Concurrent.zip(
        Array.new(5) do |i|
          Concurrent.future do 
            sync_puts "starting #{i}"
            sleep(i)
            sync_puts "done #{i}"
            i
          end
        end
      )
    end
  end  

proc_to_delay_chaining = -> do
  procs_to_delay_steps.inject do |a, b|
    f = a.respond_to?(:call) ? a.call : a
    f.chain { b.call }.flat
  end
end

sync_puts "Lazy. Nothing has happened yet...(sleep 5 to prove that)"
sleep(5)

sync_puts "Invoking composed Proc of Futures"
proc_to_delay_chaining.call.value!

# == Output ==
# 
# Lazy. Nothing has happened yet...(sleep 5 to prove that)
# Invoking composed Proc of Futures
#
# -- starting step 0
# starting 0
# done 0
# starting 1
# starting 2
# starting 3
# starting 4
# done 1
# done 2
# done 3
# done 4
#
# -- starting step 1
# starting 0
# done 0
# starting 1
# starting 2
# starting 4
# starting 3
# done 1
# done 2
# done 3
# done 4
#
# -- starting step 2
# starting 0
# starting 1
# starting 3
# starting 4
# done 0
# starting 2
# done 1
# done 2
# done 3
# done 4
#
# -- starting step 3
# starting 0
# done 0
# starting 2
# starting 4
# starting 3
# starting 1
# done 1
# done 2
# done 3
# done 4