Adapter

Make a foreign type fit the interface you expect

Adapter wraps a type with the wrong interface so it satisfies the one your code uses. PawWalk sends through a Notifier with Send(msg string) error. But the legacy mailer you must reuse only offers Deliver(text string) — different name, no error return. You can't edit it (it's a vendor package), and you don't want to rewrite every caller.

So you wrap it. A small struct embeds or holds the legacy value and exposes a Send method that translates the call. Thanks to Go's implicit interfaces, the moment your adapter has a Send(string) error method, it is a Notifier — no declaration required.