golang 闭包 函数作为参数和返回值

一、函数闭包

package main

import "fmt"

func adder() func(int) int { //函数作为返回值
	sum := 0//自由变量，每次调用都保留上次的结果
	return func(v int) int {//v是局部变量
		sum += v
		return sum
	}
}

func Fibonacci() func() int {
	a, b := 0, 1
	return func() int {
		a, b = b, a+b
		return a
	}
}

func main() {
	// a := adder() is trivial and also works.
	a := adder()
	for i := 0; i < 10; i++ {
		var s int
		s = a(i)
		fmt.Printf("0 + 1 + ... + %d = %d\n",
			i, s)
	}

	f := Fibonacci()
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
	fmt.Println(f())
}

输出：

0 + 1 + ... + 0 = 0
0 + 1 + ... + 1 = 1
0 + 1 + ... + 2 = 3
0 + 1 + ... + 3 = 6
0 + 1 + ... + 4 = 10
0 + 1 + ... + 5 = 15
0 + 1 + ... + 6 = 21
0 + 1 + ... + 7 = 28
0 + 1 + ... + 8 = 36
0 + 1 + ... + 9 = 45
1
1
2
3
5
8
13
21
34
55
89

二、函数可以实现接口

package main

import (
	"bufio"
	"fmt"
	"io"
	"strings"
)

func Fibonacci() func() int {
	a, b := 0, 1
	return func() int {
		a, b = b, a+b
		return a
	}
}


type intGen func() int

func (g intGen) Read(        
	p []byte) (n int, err error) {//函数实现了接口
	next := g()
	if next > 10000 {
		return 0, io.EOF
	}
	s := fmt.Sprintf("%d\n", next)

	// TODO: incorrect if p is too small!
	return strings.NewReader(s).Read(p)
}

func printFileContents(reader io.Reader) {//会调用上面被实现的Read函数
	scanner := bufio.NewScanner(reader)

	for scanner.Scan() {
		fmt.Println(scanner.Text())
	}
}

func main() {
	var f intGen = Fibonacci()
	printFileContents(f)
}

二、函数作为参数和返回值

package main

import (
	"net/http"
	_ "net/http/pprof"
)

type appHandler func(writer http.ResponseWriter,
	request *http.Request) error

func errWrapper(
	handler appHandler) func(
	http.ResponseWriter, *http.Request) {//函数作为参数和返回值
	return func(writer http.ResponseWriter,
		request *http.Request) {
		handler(writer, request)

	}
}

func HandleFileList(writer http.ResponseWriter,
	request *http.Request) error {
	return nil
}

func main() {
	http.HandleFunc("/",
		errWrapper(HandleFileList))

	err := http.ListenAndServe(":8888", nil)
	if err != nil {
		panic(err)
	}
}