17.函数式编程

2020-05-22  本文已影响0人  迷心迷

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

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)) } }


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

柯里化(Currying)

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

func add(_ v1: Int, _ v2: Int) -> Int { v1 + v2 }
add(10, 20)

柯里化后:
func add(_ v: Int) -> (Int) -> Int { { $0 + v } }
add(10)(20)

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

函子(Functor)

 // Array<Element>
public func map<T>(_ transform: (Element) -> T) -> Array<T>

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

适用函子(Applicative Functor)

func pure<A>(_ value: A) -> F<A>
func <*><A, B>(fn: F<(A) -> B>, value: F<A>) -> F<B>
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 Any)
func pure<A>(_ value: A) -> [A] { [value] }
func <*><A, B>(fn: [(A) -> B], value: [A]) -> [B] {
        var arr: [B] = []
        if fn.count == value.count {
                  for i 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]
print(arr)

单子(Monad)

 
func pure<A>(_ value: A) -> F<A>
func flatMap<A, B>(_ value: F<A>, _ fn: (A) -> F<B>) -> F<B>
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