深入理解函数式编程：从核心概念到实战落地

let globalCount = 0;
function add(num) {
  globalCount++; // 修改全局变量，副作用
  return num + globalCount;
}
add(1); // 返回2
add(1); // 返回3（相同输入不同输出，不纯）

function pureAdd(a, b) { return a + b; }
pureAdd(1,2); // 3，每次调用结果一致

const user = { name: "Alice", age:25 };
// 错误：直接修改原数据
// user.age = 26;
// 正确：返回新副本
const updatedUser = { ...user, age:26 };

import produce from 'immer';
const updatedUser = produce(user, draft => {
  draft.age = 26; // 看似修改原数据，实际生成新副本
});

original = (1,2,3)
new_tuple = (4,) + original[1:] # (4,2,3)，原元组不变

// 赋值给变量
const greet = name => `Hello ${name}`;
// 作为参数传入高阶函数
[1,2,3].map(x => x*2); // [2,4,6]
// 作为返回值
function createAdder(a) {
  return b => a + b;
}
const add5 = createAdder(5);
add5(3); // 8

const numbers = [1,2,3,4,5,6];
const result = numbers
  .filter(x => x%2===0) // 过滤偶数
  .map(x => x*x) // 计算平方
  .reduce((sum, x) => sum+x, 0); // 求和
console.log(result); // 56

function factorial(n, acc=1) {
  if(n<=1) return acc;
  return factorial(n-1, n*acc); // 尾递归调用
}
factorial(5); // 120

def infinite_numbers():
  n = 1
  while True:
    yield n
    n +=1
 
# 仅在迭代时计算，不会导致内存溢出
for num in infinite_numbers():
  if num>10: break
  print(num) # 1-10

function* generateNumbers() {
  let n=1;
  while(true) yield n++;
}
const gen = generateNumbers();
console.log(gen.next().value); // 1
console.log(gen.next().value); //2

const numbers = [1,2,3,4,5,6];
let sum=0;
for(let i=0;i<numbers.length;i++){
  if(numbers[i]%2===0){
    sum += numbers[i]**2;
  }
}
console.log(sum); //56

const numbers = [1,2,3,4,5,6];
const sum = numbers
  .filter(x=>x%2===0)
  .map(x=>x**2)
  .reduce((a,b)=>a+b,0);
console.log(sum); //56

const pipe = (...fns) => x => fns.reduce((acc, fn)=>fn(acc), x);
 
// 单一职责小函数
const filterEven = arr => arr.filter(x=>x%2===0);
const square = arr => arr.map(x=>x*x);
const sum = arr => arr.reduce((a,b)=>a+b,0);
 
// 组合成复杂函数
const sumOfEvenSquares = pipe(filterEven, square, sum);
sumOfEvenSquares([1,2,3,4,5,6]); //56

// 普通多参数函数
function add(a,b,c) { return a+b+c; }
// 柯里化函数
const curriedAdd = a => b => c => a+b+c;
 
// 使用方式
curriedAdd(1)(2)(3); //6
// 固定第一个参数，生成新函数
const add1 = curriedAdd(1);
add1(2)(3); //6

from functools import partial
 
def power(base, exponent):
  return base**exponent
 
# 固定指数为2，生成平方函数
square = partial(power, exponent=2)
square(3); #9
# 固定底数为2，生成2的n次方函数
powerOf2 = partial(power, base=2)
powerOf2(3); #8

const users = [
  {name:"Alice",age:17}, {name:"Bob",age:22},
  {name:"Charlie",age:19}, {name:"Diana",age:25}
];
 
const filterAdults = users => users.filter(u=>u.age>=18);
const extractFields = fields => users => 
  users.map(u=>fields.reduce((acc,f)=>({...acc, [f]:u[f]}), {}));
const avgAge = users => users.reduce((sum,u)=>sum+u.age,0)/users.length;
 
const getAdultAvgAge = pipe(filterAdults, extractFields(['name','age']), avgAge);
console.log(getAdultAvgAge(users)); //22

from functools import reduce
 
orders = [
  {"id":1, "amount":80, "status":"completed"},
  {"id":2, "amount":150, "status":"completed"},
  {"id":3, "amount":200, "status":"cancelled"},
  {"id":4, "amount":120, "status":"completed"},
]
 
def filter_valid(orders):
  return list(filter(lambda o:o["status"]=="completed" and o["amount"]>=100, orders))
def calc_taxed(orders):
  return list(map(lambda o:{"taxed":o["amount"]*1.1}, orders))
def total_sum(orders):
  return reduce((s,o)=>s+o["taxed"], orders,0.0)
 
valid_orders = filter_valid(orders)
taxed = calc_taxed(valid_orders)
print(total_sum(taxed)); //297.0

import React, {useState} from 'react';
import produce from 'immer';
 
function TodoList() {
  const [todos, setTodos] = useState([
    {id:1, text:"学习函数式编程", done:false},
    {id:2, text:"写案例", done:true}
  ]);
 
  const toggleTodo = id => {
    setTodos(produce(draft=>{
      const todo = draft.find(t=>t.id===id);
      if(todo) todo.done = !todo.done;
    }));
  };
 
  return (
    <ul>
      {todos.map(t=>(
        <li key={t.id} onClick={()=>toggleTodo(t.id)} style={{textDecoration: t.done?'line-through':'none'}}>
          {t.text}
        </li>
      ))}
    </ul>
  );
}