Python函数式编程(fn)

尽管python不是纯函数编程语言，lambda语法很不友好，但它是门多范式语言，可以方便进行扩展（如fn、pymonad、hylang），这里主要介绍fn。

fn是个python的函数式编程（Functional Programming, FP）库，极大方便了python fp。

（本准备写个教程，发现官方文档写得很详细…以下内容从 https://github/kachayev/fn.py搬运。)

1. lambda定义(“-“)

from fn import _

print (_ + 2) # "(x1) => (x1 + 2)"
print (_ + _ * _) # "(x1, x2, x3) => (x1 + (x2 * x3))"

2. lazy-seq (“Stream”)

Stream用于创建情性序列，和itertor很像，常常用于定义无穷序列。

from fn import Stream

s = Stream() << range(6) << [6,7]
assert list(s) == [0,1,2,3,4,5,6,7]

f = Stream()
fib = f << [0, 1] << map(add, f, drop(1, f))

# 创建fib无穷序列
assert list(take(10, fib)) == [0,1,1,2,3,5,8,13,21,34]
assert fib[20] == 6765
assert list(fib[30:35]) == [832040,1346269,2178309,3524578,5702887]

3. curring(柯里化)

from fn.func import curried

@curried
def sum5(a, b, c, d, e):
    return a + b + c + d + e

assert sum5(1)(2)(3)(4)(5) == sum5(1, 2, 3)(4, 5)
assert sum5(1)(2)(3)(4)(5) == sum5(1, 2, 3, 4, 5)

4. 高级函数操作（F）

fn.F用于包装并构造复杂函数

from fn import F, _
from fn.iters import filter, range

# 类似管道操作
func = F() >> (filter, _ < 6) >> sum
assert func(range(10)) == 15

最后，附图，使用fp实现dict

STOP WRITING CLASSES!!