Why are interfaces needed in Golang?

Question

In Golang, we use structs with receiver methods. everything is perfect up to here.
I\'m not sure what interfaces are, however. We define methods in structs and if we wan

Answer 1

I will show here, two interesting use cases of interfaces in Go:

1- See these two simple interfaces:

type Reader interface {
    Read(p []byte) (n int, err error)
}

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

Using these two simple interfaces you may do this interesting magic:

package main

import (
    "bufio"
    "bytes"
    "fmt"
    "io"
    "os"
    "strings"
)

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

    w := io.MultiWriter(file, os.Stdout)
    r := strings.NewReader("You'll see this string twice!!\n")
    io.Copy(w, r)

    slice := []byte{33, 34, 35, 36, 37, 38, 39, 10, 13}
    io.Copy(w, bytes.NewReader(slice)) // !"#$%&'

    buf := &bytes.Buffer{}
    io.Copy(buf, bytes.NewReader(slice))
    fmt.Println(buf.Bytes()) // [33 34 35 36 37 38 39 10 13]

    _, err = file.Seek(0, 0)
    if err != nil {
        panic(err)
    }

    r = strings.NewReader("Hello\nWorld\nThis\nis\nVery\nnice\nInterfacing.\n")
    rdr := io.MultiReader(r, file)
    scanner := bufio.NewScanner(rdr)
    for scanner.Scan() {
        fmt.Println(scanner.Text())
    }
}

Output:

You'll see this string twice!!
!"#$%&'

[33 34 35 36 37 38 39 10 13]
Hello
World
This
is
Very
nice
Interfacing.
You'll see this string twice!!
!"#$%&'

I hope this code is clear enough:
reads from string using strings.NewReader and writes concurrently to both file and os.Stdout using io.MultiWriter with just io.Copy(w, r). Then reads from slice using bytes.NewReader(slice) and writes concurrently to both file and os.Stdout. Then copy slice to the buffer io.Copy(buf, bytes.NewReader(slice)) then goto the file origin using file.Seek(0, 0) then first read from string using strings.NewReader then continue reading that file using io.MultiReader(r, file) and bufio.NewScanner and Print all of then using fmt.Println(scanner.Text()).

---

2- And this is another interesting use of interface:

package main

import "fmt"

func main() {
    i := show()
    fmt.Println(i) // 0

    i = show(1, 2, "AB", 'c', 'd', []int{1, 2, 3}, [...]int{1, 2})
    fmt.Println(i) // 7

}
func show(a ...interface{}) (count int) {
    for _, b := range a {
        if v, ok := b.(int); ok {
            fmt.Println("int: ", v)
        }
    }
    return len(a)
}

output:

0
int:  1
int:  2
7

---

And nice example to see: Explain Type Assertions in Go

Also see: Go: What's the meaning of interface{}?