Quick demonstration of how to use Go interfaces to mock third party packages that only provide concrete implementations, not interfaces.

There is an example concrete package in pkg/external/concrete. It contains some client that does two useful things: hack servers and mine bitcoins.

Our server in internal/server that we're writing for this application is interested in hacking servers, so we've imported this concrete package to do that for us. Unfortunately the concrete package has not supplied us with an interface; we can ONLY get a concrete implementation out of it.

The problem is we still want to write tests for our code. So how do we test with a client that will reach external resources if we let it?

Go interfaces to the rescue!

Go interfaces aren't like other languages where a class must inherit the interface in order to match the interface type later. Go takes the duck typing approach of "If this struct can do everything this interface asks it to, then it must be this interface."

Using this knowledge, we can actually remove the concrete.ConcreteClient type from server entirely. Instead, server will describe an interface of a client that it needs in order to function. This interface is designed to match the methods that we're interested in from concrete.ConcreteClient. Now when we create a new server, we can pass in a *concrete.ConcreteClient instance because it matches our interface. We can also create a mock implementation very easily for our unit tests. Check out the unit tests here..