What is it

Its primary job is to wrap existing implementations of such primitives, such as those in package os, into shared public interfaces that abstract the functionality, plus some other related primitives.

io.Reader

The io package specifies the io.Reader interface, which represents the read end of a stream of data.

Read populates the given byte slice with data and returns the number of bytes populated and an error value.

It returns an io.EOF error when the stream ends.

package main

import (
  "fmt"
  "io"
  "strings"
)

func main() {
  r := strings.NewReader("Hello, Reader!")
  // 8 byte at a time
  b := make([]byte, 8)

  for {
    // reads from byte arr r
    n, err := r.Read(b)

    fmt.Printf("n = %v err = %v b = %v\n", n, err, b)
    fmt.Printf("b[:n] = %q\n", b[:n])
    if err == io.EOF {
      break
    }
  }
}

io.Writer

io.Writer is an interface that wraps the basic Write method.

• Underneath the Writer is a byte array.

type Writer interface {
	Write(p []byte) (n int, err error)
}

See os section on writing to byte array.