[2023] Kotlin Tutorial Part 2 Object-Oriented and Functional Programming Chapter 13 Functional Programming Cornerstone - Higher-order Functions and Lambda Expressions 13.2 Higher-order Functions 13.2.3 Functions Used as Return Values of Another Function

[2023] Kotlin Tutorial

Part II Object Oriented and Functional Programming

Chapter 13 The Cornerstone of Functional Programming—Higher-order Functions and Lambda Expressions

Although the idea of ​​functional programming is as old as object-oriented, the computer language that supports functional programming is only a matter of recent years. These languages ​​include Swift, Python, Java 8, and C++ 11. As a new language, Kotlin also supports functional programming.

13.2 Higher-order functions

The key to functional programming is the support of higher-order functions. A function can be used as a parameter of another function, or return a value, then this function is a "higher-order function".

13.2.3 Using a function as the return value of another function

A function can be used as the return value of another function, then this function is a higher-order function. The return type of the calculate function we wrote before is (Int,Int)->Int function type,

This shows that calculate is a higher-order function.

Here's another example of using a function as the return value of another function:

// 定义计算长方形面积的函数
// 函数类型(Double, Double) → Double
fun rectangleArea(width: Double, height: Double): Double {
    
    
    return width * height;
}

// 定义计算三角形面积的函数
// 函数类型(Double, Double) → Double
fun triangleArea(bottom: Double, height: Double) = 0.5 * bottom * height

fun getArea(type: String): (Double, Double) -> Double {
    
    

    var returnFunction: (Double, Double) -> Double

    when (type) {
    
    
        "rect" -> returnFunction = ::rectangleArea
        else -> returnFunction = ::triangleArea
    }

    return returnFunction
}

fun main() {
    
    

    // 获取计算三角形面积函数
    var area: (Double, Double) -> Double = getArea("tria")
    println("底10 高15的三角形面积为: ${
      
      area(10.0, 15.0)}")

    // 获取计算长方形面积函数
    area = getArea("rect")
    println("宽10 高15的长方形面积: ${
      
      area(10.0, 15.0)}")
}

The above code fun getAreadefines the function getArea, and its return type is (Double, Double)->Double, which means that the return value is a function type.

var returnFunction: (Double, Double) -> DoubleThe code declares the returnFunction variable, explicitly specifying that its type is
(Double, Double)->Double function type.