Builds a list, using an iterator function and an initial seed value.

The iterator function accepts one argument (seed) and must always return a list with two elements ([value, nextSeed]) or False to terminate. Use a generator function, fn_generator, that uses a while loop to call the iterator function and yield the value until it returns False. Use list comprehension to return the list that is produced by the generator, using the iterator function.

def unfold(fn, seed):
  def fn_generator(val):
    while True: 
      val = fn(val[1])
      if val == False: break
      yield val[0]
  return [i for i in fn_generator([None, seed])]

f = lambda n: False if n > 50 else [-n, n + 10]
unfold(f, 10) # [-10, -20, -30, -40, -50]