c1.go

type FileSystem interface {
    Open() (io.ReadCloser, error)
    List() ([]string, error)
}

type File struct {
    name string
    path string
}

func (f *File) Open() (io.ReadCloser, error) {
    return os.Open(f.path)
}

func (f *File) List() ([]string, error) {
    return nil, fmt.Errorf("%s is a file, not a directory", f.name)
}

type Directory struct {
    name string
    path string
    files []FileSystem
}

func (d *Directory) Open() (io.ReadCloser, error) {
    return nil, fmt.Errorf("%s is a directory, not a file", d.name)
}

func (d *Directory) List() ([]string, error) {
    names := make([]string, len(d.files))
    for i, file := range d.files {
        names[i] = file.Name()
    }
    return names, nil
}