io.Writer interface

The inverse of io.Reader is io.Writer interface:

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

Compared to io.Reader, since you no need to consider io.EOF error, the process of Write method is simple:

(1) err == nil: All the data in p is written successfully;
(2) err != nil: The data in p is partially or not written at all.

Let's see an example:

package main

import (
        "log"
        "os"
)

func main() {
        f, err := os.Create("test.txt")
        if err != nil {
                log.Fatal(err)
        }
        defer f.Close()

        if _, err = f.Write([]byte{'H', 'E', 'L', 'L', 'O'}); err != nil {
                log.Fatal(err)
        }
}

After executing the program, the test.txt is created:

# cat test.txt
HELLO