Today we pick up the second half of an example we started with yesterday:

Similarly, consider this interface specification, which appears within a type declaration to define an interface called Locker:

type Locker interface {
	Lock()
	Unlock()
}

If S1 and S2 also implement

func (p T) Lock() { … }
func (p T) Unlock() { … }

they implement the Locker interface as well as the File interface.

We’ve already discussed that a single type may automatically satisfy multiple interfaces, so there’s not much to add here.

What I would like to elaborate on, though, is that an interface type doesn’t necissarily have to be named, as it is in this example. Anonymous interfaces are entirely usable as well. Let’s look at an example, building on the above code.

This code references the above Locker type in a type assertion, to see whether variable x implements the Locker interface, and if so, calls Lock() on the variable.

if locker, ok := x.(Locker); ok {
  locker.Lock()
}

We can do the same without explicitly declaring that Locker type, though:

if locker, ok := x.(interface{ Lock() }); ok {
  locker.Lock()
}

Here we inline the interface declaration anonymously (I’ve also omitted the Unlock() method, since I’m not calling it in the example.)

Quotes from The Go Programming Language Specification Version of December 15, 2022