Abstract
虽然像递归, 但是你要用functional的眼观看待
- 对js做7点限制, 从而体会函数式
- no loops
- no if
- single return
- no side-effects to outside world
- no assignments in functinos
- no arrays
- only 0 or 1 argument
Pure function, 怎么的输入就有怎样的输出，不产生side effect, 如修改外部全局变量。不依赖外部数据, 如根据全局变量的值产生输出。
- 多参数
- 链表
- first, second, …
- head, next
- 数组
- list2array
- array2list
- range
- ⭐ map
多参数逻辑
函数式的上下文在闭包中
加法为例
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| let add = (a) => (b) => a + b
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pair
有了pair的概念后就可以递归了
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| let pair = (first) => (second) => {
return {
first: first,
second: second,
}
}
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取成员
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| let fst = (pair) => pair.first
let snd = (pair) => pair.second
fst(pair(1)(2))
snd(pair(1)(2))
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链表
链表是如何递归的
利用pair做一个手动链表
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| let list = pair(1)(pair(2)(pair(3)(null)))
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list2array
有了pair的概念, 迁移到链表上就是: data, next。所以可以创建两个辅助函数data和next
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| let data = (p) => p.first
let next = (p) => p.second
let list2array = (node) => {
let result = []
while (node !== null) {
result.push(data(node))
node = next(node)
}
return result
}
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array2list
创node, 接node
不够我们这里要从尾到头创建: 递归的尽头是next
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| let array2list = (a) => {
let xs = Array.from(a).reverse()
let node = null
for (let i=0; i<xs.length; i++) {
node = pair(xs[i])(node)
}
return node;
}
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range
给定一个范围自动创建一range的链表
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| let range = (low) => (high) =>
low > high ?
null : pair(low)(range(low+1)(high))
list2array(range(1)(100))
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map
map(f)(list), 将f作用于list中的所有元素然后返回新list
之所以抽象成list是因为我们想尽可能用函数式的方式遍历
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| let map = (f) => (list) => {
return list === null
? null
: pair(f(data(list))) (map(f)(next(list)))
}
list2array(map((x) => x*2)(range(1)(10)))
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总结
- 多参数传递的模式:
- 不管三七二十一, 先把函数参数保存到闭包中再说
- 也就相当于
(arg1, arg2, ...)变成了(arg1)(arg2)(...)
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| let pair = (first) => (second) => {
return {
first: first,
second: second,
}
}
let fst = (pair) => pair.first
let snd = (pair) => pair.second
let list = pair(1)(pair(2)(pair(3)(null)))
let data = (p) => p.first
let next = (p) => p.second
let list2array = (node) => {
let result = []
while (node !== null) {
result.push(data(node))
node = next(node)
}
return result
}
let array2list = (a) => {
let xs = Array.from(a).reverse()
let node = null
for (let i=0; i<xs.length; i++) {
node = pair(xs[i])(node)
}
return node;
}
let range = (low) => (high) =>
low > high ?
null : pair(low)(range(low+1)(high))
list2array(range(1)(100))
let map = (f) => (list) => {
return list == null
? null
: pair(f(data(list))) (map(f)(next(list)))
}
list2array(map((x) => x*2)(range(1)(10)))
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