Struct doesn't implement interface even if it has the same functions

Question

I don't know what is the reason of following compilation error. I will appreciate any help.

./router.go:190: cannot use listener (type webhooklistener.MyListener) as type webhook.Listener in field value:
    webhooklistener.MyListener does not implement webhook.Listener (missing webhook.handle method)
        have webhooklistener.handle()
        want webhook.handle()

Client:

package webhook

type Listener interface {
    handle()
}

type Client struct {
    Listener Listener
}

Listener:

package webhooklistener

type MyListener struct {
}

func (ll MyListener) handle() {

}

Router:

listener := webhooklistener.MyListener{}
client := webhook.Client{listener} // COMPILATION ERROR

Answer 1

webhook.Listener's only method is unexported, so only identifiers in that package can implement it. If you want types in other packages to be able to implement it, you need to make it exported:

type Listener interface {
    Handle()
}