How do io.Reader and io.Writer demonstrate Go's interface design philosophy?

// io.Reader and io.Writer — the entire interface
type Reader interface { Read(p []byte) (n int, err error) }
type Writer interface { Write(p []byte) (n int, err error) }

// Composition: ReadWriter, ReadCloser, WriteCloser
type ReadWriter  interface { Reader; Writer }
type ReadCloser  interface { Reader; Closer }
type WriteCloser interface { Writer; Closer }

// Any function accepting io.Reader can work with ALL of these:
func countBytes(r io.Reader) (int64, error) {
    return io.Copy(io.Discard, r)
}

// Works identically for:
f, _ := os.Open("data.txt")
countBytes(f)                          // *os.File
countBytes(strings.NewReader("hello")) // *strings.Reader
countBytes(bytes.NewReader(data))       // *bytes.Reader
countBytes(resp.Body)                   // http.Response.Body
countBytes(gzip.NewReader(f))            // *gzip.Reader

// Composing transforms: gzip decompress → buffered read → count
f2, _ := os.Open("archive.gz")
gzr, _ := gzip.NewReader(f2)
buf := bufio.NewReader(gzr)
count, _ := countBytes(buf)
// Each layer is transparent to countBytes — it just calls Read()

// Writing to multiple destinations simultaneously
var buf2 bytes.Buffer
multi := io.MultiWriter(&buf2, os.Stdout)
fmt.Fprintln(multi, "hello") // writes to both buf2 and stdout