Functions as values and types

package main

import "fmt"

type testInt func(int) bool // define a function type of variable

/*
	它的执行效果就是把 f 换成实际传入的函数，比如第 29 行展开大概是这样：
		var result []int
		for _, value := range slice {
			if isOdd(value) {
				result = append(result, value)
			}
		}
		return result
*/

func isOdd(integer int) bool {
	return integer%2 != 0
}

func isEven(integer int) bool {
	return integer%2 == 0
}

// pass the function `f` as an argument to another function
func filter(slice []int, f testInt) []int {
	var result []int
	for _, value := range slice {
		if f(value) {
			result = append(result, value)
		}
	}
	return result
}

var slice = []int{1, 2, 3, 4, 5, 7}

func main() {
	odd := filter(slice, isOdd)
	even := filter(slice, isEven)

	fmt.Println("slice = ", slice)
	fmt.Println("Odd elements of slice are: ", odd)
	fmt.Println("Even elements of slice are: ", even)
}