strings.Map(func(r rune) rune { return r + 1 }, "HAL-9000")

// squares返回一个匿名函数。
// 该匿名函数每次被调用时都会返回下一个数的平方。
func squares() func() int {
	var x int
	return func() int {
		x++
		return x * x
	}
}
func main() {
	f := squares()
	fmt.Println(f()) // "1"
	fmt.Println(f()) // "4"
	fmt.Println(f()) // "9"
	fmt.Println(f()) // "16"
}

// prereqs记录了每个课程的前置课程
var prereqs = map[string][]string{
	"algorithms": {"data structures"},
	"calculus": {"linear algebra"},
	"compilers": {
		"data structures",
		"formal languages",
		"computer organization",
	},
	"data structures":       {"discrete math"},
	"databases":             {"data structures"},
	"discrete math":         {"intro to programming"},
	"formal languages":      {"discrete math"},
	"networks":              {"operating systems"},
	"operating systems":     {"data structures", "computer organization"},
	"programming languages": {"data structures", "computer organization"},
}

func main() {
	for i, course := range topoSort(prereqs) {
		fmt.Printf("%d:\t%s\n", i+1, course)
	}
}

func topoSort(m map[string][]string) []string {
	var order []string
	seen := make(map[string]bool)
	var visitAll func(items []string)
	visitAll = func(items []string) {
		for _, item := range items {
			if !seen[item] {
				seen[item] = true
				visitAll(m[item])
				order = append(order, item)
			}
		}
	}
	var keys []string
	for key := range m {
		keys = append(keys, key)
	}
	sort.Strings(keys)
	visitAll(keys)
	return order
}

visitAll := func(items []string) {
	// ...
	visitAll(m[item]) // compile error: undefined: visitAll
	// ...
}

// Package links provides a link-extraction function.
package links
import (
	"fmt"
	"net/http"
	"golang.org/x/net/html"
)
// Extract makes an HTTP GET request to the specified URL, parses
// the response as HTML, and returns the links in the HTML document.
func Extract(url string) ([]string, error) {
	resp, err := http.Get(url)
	if err != nil {
		return nil, err
	}
	if resp.StatusCode != http.StatusOK {
	resp.Body.Close()
		return nil, fmt.Errorf("getting %s: %s", url, resp.Status)
	}
	doc, err := html.Parse(resp.Body)
	resp.Body.Close()
	if err != nil {
		return nil, fmt.Errorf("parsing %s as HTML: %v", url, err)
	}
	var links []string
	visitNode := func(n *html.Node) {
		if n.Type == html.ElementNode && n.Data == "a" {
			for _, a := range n.Attr {
				if a.Key != "href" {
					continue
				}
				link, err := resp.Request.URL.Parse(a.Val)
				if err != nil {
					continue // ignore bad URLs
				}
				links = append(links, link.String())
			}
		}
	}
	forEachNode(doc, visitNode, nil)
	return links, nil
}

// breadthFirst calls f for each item in the worklist.
// Any items returned by f are added to the worklist.
// f is called at most once for each item.
func breadthFirst(f func(item string) []string, worklist []string) {
	seen := make(map[string]bool)
	for len(worklist) > 0 {
		items := worklist
		worklist = nil
		for _, item := range items {
			if !seen[item] {
				seen[item] = true
				worklist = append(worklist, f(item)...)
			}
		}
	}
}

func crawl(url string) []string {
	fmt.Println(url)
	list, err := links.Extract(url)
	if err != nil {
		log.Print(err)
	}
	return list
}

func main() {
	// Crawl the web breadth-first,
	// starting from the command-line arguments.
	breadthFirst(crawl, os.Args[1:])
}

var rmdirs []func()
for _, d := range tempDirs() {
	dir := d // NOTE: necessary!
	os.MkdirAll(dir, 0755) // creates parent directories too
	rmdirs = append(rmdirs, func() {
		os.RemoveAll(dir)
	})
}
// ...do some work…
for _, rmdir := range rmdirs {
	rmdir() // clean up
}

var rmdirs []func()
for _, dir := range tempDirs() {
	os.MkdirAll(dir, 0755)
	rmdirs = append(rmdirs, func() {
		os.RemoveAll(dir) // NOTE: incorrect!
	})
}

for _, dir := range tempDirs() {
	dir := dir // declares inner dir, initialized to outer dir
	// ...
}

var rmdirs []func()
dirs := tempDirs()
for i := 0; i < len(dirs); i++ {
	os.MkdirAll(dirs[i], 0755) // OK
	rmdirs = append(rmdirs, func() {
		os.RemoveAll(dirs[i]) // NOTE: incorrect!
	})
}