a.rb

def pmap(enum)
  return to_enum(:pmap, enum) unless block_given?
  enum.each_with_index.map { |e,i| Thread.new { sleep(Random.rand); p e; yield ({index: i, value:e }) } }.map(&:value)
end

p to_enum(:pmap, 1..10).sort_by { |hash| hash[:index] }.map { |hash| hash[:value] }
#p pmap(1..10) { |x| x }