A recursion version of parallel reduce similar to the iterative version

reduce.py

def reduce(arr):
  def impl(i, s):
    if i >= len(arr):
      return 0
    elif i + s >= len(arr):
      return arr[i]
    
    a = impl(i, s * 2)  # spawn
    b = impl(i + s, s * 2)
    return a + b
  
  return impl(0, 1)

print(reduce([1, 2, 3, 4, 5, 7, 8, 9, 10]))