A Tour of Go Interfaces are satisfied implicitly

A type implements an interface by implementing the methods.

There is no explicit declaration of intent.

Implicit interfaces decouple implementation packages from the packages that define the interfaces: neither depends on the other.

It also encourages the definition of precise interfaces, because you don't have to find every implementation and tag it with the new interface name.

Package io defines Reader and Writer; you don't have to.

package main

import (
    "fmt"
    "os"
)

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

type ReadWriter interface {
    Reader
    Writer
}

func main() {
    var w Writer

    //os.Stdout implements Writer
    w = os.Stdout

    fmt.Fprintf(w, "hello, writer\n")
}