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

io.Reader and io.Writer are the most influential interfaces in the Go standard library. They each have exactly one method, yet they model an enormous variety of data sources and sinks — files, network connections, byte buffers, compression streams, crypto pipes, and more.

// 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

The lesson: small interfaces with well-chosen methods compose powerfully. By accepting io.Reader instead of *os.File, a function becomes universally useful. Each layer in the stack (file, gzip, bufio) wraps the previous one, extending behaviour without modifying the original — the Decorator pattern in Go.

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