Interfaces

Relax. We'll talk through this in plain words — no textbook voice.

Interface is a contract that defines method behavior. In Go, you don't need to explicitly write "implements" for an interface. If a type has all the methods in the interface, it automatically satisfies it.

package main

import "fmt"

type Notifier interface {
    Notify(message string)
}

type EmailNotifier struct{}

func (EmailNotifier) Notify(message string) {
    fmt.Println("Email:", message)
}

func sendAlert(n Notifier) {
    n.Notify("Server is running")
}

func main() {
    email := EmailNotifier{}
    sendAlert(email)
}

EmailNotifier has a Notify method, so it satisfies the Notifier interface. That's why sendAlert can accept an EmailNotifier.

You should see

Email: Server is running

Info

Using interfaces lets a function work with several types that share the same behavior, instead of being tied to one specific concrete type.

Easy traps

If the method signature doesn't exactly match what the interface expects, it won't satisfy it. The parameter types, return type, and method name all need to match exactly.