Swift笔记 - 16.函数式编程

Swift笔记 - 16.函数式编程

2022-03-28 16:00

Swift笔记 - 16.函数式编程

函数式编程

  • Array的常见函数式操作
var arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
var arr2 = arr.map { $0 * 2 }
// [2, 4]
var arr3 = arr.filter {$0 % 2 == 0 }
// 10
var arr4 = arr.reduce (0) { $0 + $1 }
// 10
var arr5 = arr. reduce (0, +)

func double(_ i: Int) -> Int { i * 2 }
var arr = [1, 2, 3, 4]
// [2, 4, 6, 8]
print(arr.map (double) )

var arr = [1, 2, 3]
// [1], (2, 2], [3, 3, 3]]
var arr2 = arr.map { Array.init(repeating: $0, count: $0) }
// [1, 2, 2, 3, 3, 3]
var arr3 = arr.flatMap { Array.init(repeating: $0, count: $0) }
var arr = ["123", "test", "jack", "-30"]
// [Optional(123), nil, nil, Optional(-30)]
var arr2 = arr.map{ Int($0) }
// [123, -30]
var arr3 = arr.compactMap { Int($0) }

// 使用reduce实现map，filter的功能
var arr = [1,2,3,4]
// [2, 4, 6, 8]
print(arr.map{ $0 * 2 }
print(arr.reduce([]){ $0 + [ $1 * 2] })
// [2, 4]
print(arr.filter { $0 % 2 == 0 })
print (arr.reduce([]) { $1 %2 == 0 ? $0 + [$1] : $0})
  • lazy优化
let arr = [1, 2, 3]
let result = arr.lazy.map
(i: Int) -> Int in
print ("mapping \(i)")
return i * 2
}
print ("begin------")
print ("mapped", result[0])
print ("mapped", result(1])
print ("mapped", result[2])
print ("end------")

//output:
begin------
mapping 1
mapped 2
mapping 2
mapped 4
mapping 3
mapped 6
end------
  • optional的map与flatMap
var num1: Int? = 10
// Optional(20)
var num2 = num1.map ( $0 * 2 )

var num3: Int? = nil
// nil
var num4 = num3.map { $0 * 2 }

var num1: Int? = 10
// Optional(Optional(20))
var num2 = num1.map(Optional.some($0 * 2) }
// Optional(20)
var num3 = num1.flatMap ( Optional.some($0 * 2) }

var num1: Int? = 10
var num2 = (num1 != nil) ? (num1! + 10) : nil
var num3 = num1.map { $0 + 10 }

var fmt = DateFormatter()
ft.dateFormat = "yyyy-MM-dd"
var str: String? = "2011-09-10"
// old
var datel = str != nil ? fmt.date(from: str!) : nil
// new
var date2 = str.flatMap(fmt.date)

var score: Int? = 98
// old
var str1 = score != nil ? "socre is \(score!)": "No score"
// new
var str2 = score.map( "score is \($0)" } ?? "No score"

struct Person {
    var name: String
    var age: Int
}
var items = [
    Person(name: "jack", age: 20),
    Person(name: "rose", age: 21),
    Person(name: "kate", age: 22)
]
// old
func getPerson1(_ name: String) -> Person? {
    let index = items.firstIndex { $0.name == name }
    return index != nil ? items[index!] : nil
}
// new
func getPerson2(_ name: String) -> Person? {
    return items.firstIndex { $0.name == name }.map { items [$0] }
}

struct Person {
    var name: String
    var age: Int
    init?(_ json: [String: Any]) {
        guard let name = json["name"] as? String,
              let age = json["age"] as? Int else {
            return nil
        }
        self.name = name
        self. age = age
    }
}
var json: Dictionary? = ("name" : "Jack", "age" : 10]
// old
var p1 = json != nil ? Person(json!) : nil
// new
var p2 = json.flatMap(Person.init)

  • 函数式编程

    • 函数式塘程（FuntionalProgramming，简称FP）是一种编程范式，也就是如何编写程序的方法论
      • 主要思想：把计算过程尽量分解成一系列可复用西数的调用
      • 主要特征：西数是“第一等公民”
        • 函数与其他数据类型一样的地位，可以赋值给其他变量，也可以作为函数参数、函数返回值
    • 函数式编程最早出现在LISP语言，绝大部分的现代编程语言也对函数式编程做了不同程度的支持，比如
      • Haskell、JavaScript、Python、Swift、Kotlin、Scala等
    • 函数式编程中几个常用的概念
      • Higher-Order Function、Function Currying
      • Functor、Applicative Functor、Monad
    • 参考资料

  • 函数式写法

func add(_ v: Int) -> (Int) -> Int { { $0 + v } }
func sub(_ v: Int) -> (Int) -> Int ‹ { $0 - v } }
func multiple(_ v: Int) -> (Int) -> Int { { $0 * v } }
func divide(_ v: Int) -> (Int) -> Int { $0 / v } }
func mod(_ v: Int) -> (Int) -> Int { { $0 % v }

infix operator >>> : AdditionPrecedence
func >>><A, B, C>(_ f1: @escaping (A) -> B,
                  _ f2: @escaping (B) -> C) -> (A) -> C { { f2(f1($0)) } }
  • 高阶函数
    • 高阶函数是至少满足下列一个条件的函数：
      • 接受一个或多个西数作为输入（map、filter、reduce等）
      • 返回一个函数
  • FP中到处都是高阶函数
func add(_ v: Int) -> (Int) -> Int { { $0 + v } }

  • 柯里化

    • 什么是柯里化？

      • 将一个接受多参数的函数变换为一系列只接受单个参数的函数

    • Array、Optional的map方法接收的参数就是一个柯里化函数

    func add1(_ v1: Int, _ v2: Int) -> Int { v1 + v2 }
func add2(_ v1: Int, _ v2: Int, _ v3: Int) -> Int { v1 + v2 + v3 }

func currying<A, B, C, D>(_ fn: @escaping (A, B) -> C) 
    -> (B) -> (A) -> C {
      { b in { a in fn(a, b) } }
}
func currying<A, B, C, D>(_ fn: @escaping (A, B, C) -> D)
    -> (C) -> (B) -> (A) -> D {
      { c in { b in ( a in fn(a, b, c) }} }
}

let curriedAdd1 = currying(add1)
print(curriedAdd1(10)(20))
let curriedAdd2 = curryinq(add2)
print(curriedAdd2(10)(20) (30))

func add(_ v1: Int, _ v2: Int) -> Int { v1 + v2 }
func sub(_ v1: Int, _ v2: Int) -> Int { v1 - v2 }
func multiple(_ v1: Int, _ v2: Int) -> Int { v1 * v2 }
func divide(_ vl: Int, _ v2: Int) -> Int { v1 / v2 }
func mod(_ v1: Int, _ v2: Int) -> Int { v1 % v2 }

prefix func ~<A, B, C>(_ fn: @escaping (A, B) -> C)
      -> (B) -> (A) -> C { { b in (a in fn(a, b) } } }

infix operator >>> : AdditionPrecedence
func >>><A, B, C>(_ f1: @escaping (A) -> B,
                  _ f2: @escaping (B) -> C) -> (A) -> C { { f2(f1($0)) } }

var num = 1
var fn = (~add)(3) >>> (~multiple)(5) >>> (~sub)(1) >>> (~mod)(10) >>> (~divide)(2)
fn(num)

    func add2(_ v1: Int, _ v2: Int, _ v3: Int) -> Int { v1 + v2 + v3 )

func add(_ v1: Int, _ v2: Int) -> Int { v1 + v2 } 
func sub(_ v1: Int, _ v2: Int) -> Int { v1 - v2 } 
func multiplei(_ v1: Int, _ v2: Int) -> Int { v1 * v2 }
func divide(_ v1: Int, _ v2: Int) -> Int { v1 / v2 }
func mod(_ v1: Int, _ v2: Int) -> Int { v1 % v2 }

prefix func ~<A, B, C>(_ fn: @escaping (A, B) -> C) -> (B) -> (A) -> C {
    { b in { a in fn(a, b) } }  
}

infix operator »›> : AdditionPrecedence
func >>> <A, B, C>(_ f1: @escaping (A) -> B, _ f2: @escaping (B) -> C)
        -> (A) -> C { { f2(f1($0)) } } 

let fn = (~add]({{site.url}}/3) >>> (~multiple)(5) >>> (~sub)(1) >>> (~mod)(10) >>> (~divide)(2)
print(fn(1))

func add2(_ v1: Int, _v2: Int, _ v3: Int) -> Int { v1 - v2 + v3 }

prefix func ~<A, B, C, D>(_ fn: @escaping(A, B, C) -> D)
    -> (C) -> (B) -> (A) -> D {
      	{ c in { b in { a in fn(a, b, c) } } }
}

print((~add2)(30)(20)(10))

  • 函子(Functor)

    • 像Array、Optional这样支持map运算的类型，称为西子(Functor)
    // Array<Element>
public func map<T>(_ transform: (Element) -> T) -> Array<T>

// Optional<Wrapped>
public func map<U>(_ transform: (Wrapped) -> U) -> Optional«l>

  • 适用函子

    • 对任意一个西子F，如果能支持以下运算，该函子就是一个适用函子
    func pure<A>(_ value: A) -> F<A>
func<x><A,B>(fn:F<(A)->B>,value:F<A>)一>F<B>
    • Optional可以成为适用函子
    func pure<A> (_ value: A) -> A? { value }
infix operator <*> : AdditionPrecedence
func <*><A, B> (fn: ((A) -> B)?, value: A?) -> B? {
    guard let f = fn, let v = value else { return nil }
    return f(v)
}

var value: Int? = 10
var fn: ((Int) -> Int)? = { $0 * 2}
// Optional(20)
print(fn <*> value as Anv)

  • Array可以成为适用函子

    func pure<A>(_ value: A) -> [A] ( [value] }
func <><A, B>(fn: [(A) -> B], value: [A]) -> [B] {
fvar arr: (B) = 0
if fn. count == value.count {
    for 1 in fn.startIndex..<fn.endIndex {
        arr.append(fn[i)(value(i)))
    }
return arr


// [10]
print(pure(10))
var arr = [{$0 * 2}, {$0 + 10}, {$0 - 5}] <*> [1, 2, 3]
// [2, 12, -2]

  • 单子(Monad)

    • 对任意一个类型F，如果能支持以下运算，那么就可以称为是一个单子(Monad)
    func pure<A>(_ value: A) -> F<A>
func flatMap<A, B>(_ value: F<A>, _ fn: (A) -> F<B>) -> F<B>
    • 很显然,Array、Optional都是单子

继续阅读

相关文章