errgroup

JustErrors illustrates the use of a Group in place of a sync.WaitGroup to simplify goroutine counting and error handling. This example is derived from the sync.WaitGroup example at https://golang.org/pkg/sync/#example_WaitGroup.

g := New()
var urls = []string{
	"http://www.golang.org/",
	"http://www.google.com/",
	"http://www.somestupidname.com/",
}
for _, url := range urls {
	// Launch a goroutine to fetch the URL.
	url := url // https://golang.org/doc/faq#closures_and_goroutines
	g.Go(func() error {
		// Fetch the URL.
		resp, err := http.Get(url)
		if err == nil {
			resp.Body.Close()
		}
		return err
	})
}
// Wait for all HTTP fetches to complete.
if err := g.Wait(); err == nil {
	fmt.Println("Successfully fetched all URLs.")
}

Output:

Parallel illustrates the use of a Group for synchronizing a simple parallel task: the "Google Search 2.0" function from https://talks.golang.org/2012/concurrency.slide#46, augmented with a Context and error-handling.

Google := func(ctx context.Context, query string) ([]Result, error) {
	g, ctx := WithContext(ctx)

	searches := []Search{Web, Image, Video}
	results := make([]Result, len(searches))
	for i, search := range searches {
		i, search := i, search // https://golang.org/doc/faq#closures_and_goroutines
		g.Go(func() error {
			result, err := search(ctx, query)
			if err == nil {
				results[i] = result
			}
			return err
		})
	}
	if err := g.Wait(); err != nil {
		return nil, err
	}
	return results, nil
}

results, err := Google(context.Background(), "golang")
if err != nil {
	fmt.Fprintln(os.Stderr, err)
	return
}
for _, result := range results {
	fmt.Println(result)
}

Output:

web result for "golang"
image result for "golang"
video result for "golang"