def done(x):
    return (type(x) != type(lambda: None))

  def allof(f, *xs):
    return all(map(f, xs))

  def isnt(f):
    return lambda x: not f(x)

  def myprint(xs):
    print("\t-> ", "\t".join([str(x) if done(x) else "_" for x in xs]))

  def lazy(f, *deps):
    "Evaluate f(xs), only if dependencies (listed by index in xs) are resolved."
    def lazy_f(xs):
      if allof(done, *[xs[d] for d in deps]):
        return f(xs)
      else:
        return lazy(f, *deps)
    return lazy_f

  def loeb_unsafe(fs):
    "Loeb for lists, but doesn't detect dependency cycles."
    xs = fs[:]

    # THE IMPORTANT PART!
    while not allof(done, *xs):
      # 'if done(x) is special for Python: convert function to value, since
      # Python doesn't know how to evaluate/call a value like Haskell does.
      xs = [x if done(x) else x(xs) for x in xs]
    return xs

  def loeb_safe(fs):
    "Loeb for lists, stops when it detects a dependency cycle."
    xs = fs[:]

    # THE IMPORTANT PART!
    while not allof(done, *xs):
      myprint(xs)
      # 'if done(x) is special for Python: convert function to value, since
      # Python doesn't know how to evaluate/call a value like Haskell does.
      new_xs = [x if done(x) else x(xs) for x in xs]
      if list(filter(done, new_xs)) == list(filter(done, xs)):
        print("CYCLE DETECTED!")
        break
      xs = new_xs
    return xs

  fs = [
    lazy(lambda xs: xs[1] + xs[2], 1, 2),
    lazy(lambda xs: xs[3] + 1, 3),
    lazy(lambda xs: xs[1] + 1, 1),
    1,
    lazy(lambda xs: xs[5] + 1, 5),
    # unsafe!
    # lazy(lambda xs: xs[4] + 1, 4),
    lambda xs: 100,
    200
  ]
  
  print("safe:")
  myprint(loeb_safe(fs))
  print("\nunsafe:")
  myprint(loeb_unsafe(fs))

  safe:
    ->  _   _   _   1   _   _   200
    ->  _   2   _   1   _   100 200
    ->  _   2   3   1   101 100 200
    ->  5   2   3   1   101 100 200

  unsafe:
    ->  5   2   3   1   101 100 200