err := flowmatic.Do(
    func() error {
        return doThingA(),
    },
    func() error {
        return doThingB(),
    },
    func() error {
        return doThingC(),
    })

var wg sync.WaitGroup
var errs []error
errChan := make(chan error)

wg.Add(3)
go func() {
    defer wg.Done()
    if err := doThingA(); err != nil {
        errChan <- err
    }
}()
go func() {
    defer wg.Done()
    if err := doThingB(); err != nil {
        errChan <- err
    }
}()
go func() {
    defer wg.Done()
    if err := doThingC(); err != nil {
        errChan <- err
    }
}()

go func() {
    wg.Wait()
    close(errChan)
}()

for err := range errChan {
    errs = append(errs, err)
}

err := errors.Join(errs...)

things := []someType{thingA, thingB, thingC}

err := flowmatic.Each(numWorkers, things,
    func(thing someType) error {
        foo := thing.Frobincate()
        return foo.DoSomething()
    })

things := []someType{thingA, thingB, thingC}

work := make(chan someType)
errs := make(chan error)

for i := 0; i < numWorkers; i++ {
    go func() {
        for thing := range work {
            // Omitted: panic handling!
            foo := thing.Frobincate()
            errs <- foo.DoSomething()
        }
    }()
}

go func() {
    for _, thing := range things {
            work <- thing
    }

    close(tasks)
}()

var collectedErrs []error
for i := 0; i < len(things); i++ {
    collectedErrs = append(collectedErrs, <-errs)
}

err := errors.Join(collectedErrs...)

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

func Google(ctx context.Context, query string) ([]Result, error) {
	searches := []Search{Web, Image, Video}
	return flowmatic.Map(ctx, flowmatic.MaxProcs, searches,
		func(ctx context.Context, search Search) (Result, error) {
			return search(ctx, query)
		})
}

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

func main() {
	m, err := MD5All(context.Background(), ".")
	if err != nil {
		log.Fatal(err)
	}

	for k, sum := range m {
		fmt.Printf("%s:\t%x\n", k, sum)
	}
}

// MD5All reads all the files in the file tree rooted at root
// and returns a map from file path to the MD5 sum of the file's contents.
// If the directory walk fails or any read operation fails,
// MD5All returns an error.
func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) {
	// Make a pool of 20 digesters
	in, out := flowmatic.TaskPool(20, digest)

	m := make(map[string][md5.Size]byte)
	// Open two goroutines:
	// one for reading file names by walking the filesystem
	// one for recording results from the digesters in a map
	err := flowmatic.All(ctx,
		func(ctx context.Context) error {
			return walkFilesystem(ctx, root, in)
		},
		func(ctx context.Context) error {
			for r := range out {
				if r.Err != nil {
					return r.Err
				}
				m[r.In] = *r.Out
			}
			return nil
		},
	)

	return m, err
}

func walkFilesystem(ctx context.Context, root string, in chan<- string) error {
	defer close(in)

	return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
		if err != nil {
			return err
		}
		if !info.Mode().IsRegular() {
			return nil
		}
		select {
		case in <- path:
		case <-ctx.Done():
			return ctx.Err()
		}

		return nil
	})
}

func digest(path string) (*[md5.Size]byte, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	hash := md5.Sum(data)
	return &hash, nil
}

type result struct {
	path string
	sum  [md5.Size]byte
}

// MD5All reads all the files in the file tree rooted at root and returns a map
// from file path to the MD5 sum of the file's contents. If the directory walk
// fails or any read operation fails, MD5All returns an error.
func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) {
	// ctx is canceled when g.Wait() returns. When this version of MD5All returns
	// - even in case of error! - we know that all of the goroutines have finished
	// and the memory they were using can be garbage-collected.
	g, ctx := errgroup.WithContext(ctx)
	paths := make(chan string)

	g.Go(func() error {
		defer close(paths)
		return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
			if err != nil {
				return err
			}
			if !info.Mode().IsRegular() {
				return nil
			}
			select {
			case paths <- path:
			case <-ctx.Done():
				return ctx.Err()
			}
			return nil
		})
	})

	// Start a fixed number of goroutines to read and digest files.
	c := make(chan result)
	const numDigesters = 20
	for i := 0; i < numDigesters; i++ {
		g.Go(func() error {
			for path := range paths {
				data, err := ioutil.ReadFile(path)
				if err != nil {
					return err
				}
				select {
				case c <- result{path, md5.Sum(data)}:
				case <-ctx.Done():
					return ctx.Err()
				}
			}
			return nil
		})
	}
	go func() {
		g.Wait()
		close(c)
	}()

	m := make(map[string][md5.Size]byte)
	for r := range c {
		m[r.path] = r.sum
	}
	// Check whether any of the goroutines failed. Since g is accumulating the
	// errors, we don't need to send them (or check for them) in the individual
	// results sent on the channel.
	if err := g.Wait(); err != nil {
		return nil, err
	}
	return m, nil
}