My answer for: https://go-tour-jp.appspot.com/concurrency/10

exercise-web-crawler.go

package main

import (
	"fmt"
	"time"
)

type Fetcher interface {
	// Fetch returns the body of URL and
	// a slice of URLs found on that page.
	Fetch(url string) FetchResult
}

type FetchResult struct {
	body string
	urls []string
	err  error
}

type Crawler struct {
	fetcher Fetcher
	results map[string]chan FetchResult
}

func (crawler *Crawler) Crawl(url string, depth int) {
	if depth <= 0 {
		return
	}
	if _, ok := crawler.results[url]; ok {
		// do nothing
	} else {
		crawler.results[url] = make(chan FetchResult, 1)
		crawler.results[url] <- crawler.fetcher.Fetch(url)
	}
	result := <-crawler.results[url]
	if result.err != nil {
		fmt.Println(result.err)
		return
	}
	fmt.Printf("found: %s %q\n", url, result.body)
	for _, u := range result.urls {
		go crawler.Crawl(u, depth-1)
	}
	return
}

func main() {
	crawler := Crawler{fetcher, map[string]chan FetchResult{}}
	crawler.Crawl("https://golang.org/", 4)
	time.Sleep(50 * time.Millisecond)
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
	body string
	urls []string
}

func (f fakeFetcher) Fetch(url string) FetchResult {
	if res, ok := f[url]; ok {
		return FetchResult{res.body, res.urls, nil}
	}
	return FetchResult{"", nil, fmt.Errorf("not found: %s", url)}
}

// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
	"https://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"https://golang.org/pkg/",
			"https://golang.org/cmd/",
		},
	},
	"https://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"https://golang.org/",
			"https://golang.org/cmd/",
			"https://golang.org/pkg/fmt/",
			"https://golang.org/pkg/os/",
		},
	},
	"https://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
	"https://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
}