How to indicate in Go that a structure implements a certain interface?

In Go, a struct implicitly implements an interface if it contains all the methods declared in that interface. There is no need to explicitly declare that a struct implements an interface — it is enough to implement the corresponding methods.

Example:

package main
import "fmt"

type Speaker interface {
    Speak() string
}

type Dog struct {}

func (d Dog) Speak() string {
    return "Woof!"
}

func main() {
    var s Speaker = Dog{} // Dog implements the Speaker interface
    fmt.Println(s.Speak())
}

Here, Dog implements the Speak() method, so it automatically implements the Speaker interface.