yield 的学习，模拟无限数据 ::-- ZoomQuiet [DateTime(2007-10-24T04:15:39Z)] TableOfContents

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1. yield 的学习，模拟无限数据

对haskell我最喜爱的特性是惰性计算和无限数据结构，以及函数组合。 这些特性在python中是否可以模拟出来呢？ 今天作了一些尝试

python中提供了yield 和 decorators, 应当可以模拟出。

1.1. 尝试: 无限数据结构

haskell: [1..]

Hugs> take 5 [1..]
[1,2,3,4,5]

Python:

>>> def inf_list1():
i = 0
while 1:
i += 1
yield i

>>> def take(n, it):
cnt = 0
for i in it():
cnt += 1
if cnt > n: break
yield i

>>> take(5, m)

>>> [i for i in take(5, inf_list1)]
[1, 2, 3, 4, 5]

Python 的 take 也实现为一个 generator ，这样的好处是可以串起来执行，比如，我要实现 haskell中的： Hugs> drop 3 $ take 5 [1..] [4,5]

• 那么再实现一个 drop 函数：

>>> def drop(n, it):
t = 0
for i in it():
t += 1
if t > n:
yield i

>>> def m10():
for i in range(10):
yield i

>>> [i for i in drop(3, m10)]
[3, 4, 5, 6, 7, 8, 9]
>>> [i for i in take(5, m10)]
[0, 1, 2, 3, 4]
>>>

• 不过，在组合 take 和 drop 的时候遇到了麻烦

>>> drop(3, take(5, m10))

>>> [i for i in drop(3, take(5, m10))]

Traceback (most recent call last):
File "", line 1, in -toplevel-
[i for i in drop(3, take(5, m10))]
File "", line 3, in drop
for i in it():
TypeError: 'generator' object is not callable
>>>

• 修改下 drop, take 的定义如下：

>>> def take(n, it):
cnt = 0
if callable(it): it=it()
for i in it:
cnt += 1
if cnt > n: break
yield i
>>> def drop(n, it):
cnt = 0
if callable(it): it = it()
for i in it:
cnt += 1
if cnt > n:
yield i

>>> [i for i in take(2, drop(3, take(7, inf_list1)))]
[4, 5]
>>>

1.2. 对应 haskell

Hugs> take 2 $ drop 3 $ take 7 [1..]
[4,5]
Hugs>

基本达到目的。

1.3. 反馈

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