What is an interface in Go? How does duck typing work? Is it possible to check the implementation of an interface at compile time?

In Go, an interface is a set of methods that a type must implement to be considered an implementation of that interface. Interfaces in Go are implemented implicitly: if a type has all the methods of an interface, it automatically implements it — this is an example of duck typing ("if it walks like a duck and quacks like a duck, it’s a duck").

Duck typing allows writing more flexible and extensible code without explicitly declaring implementation of an interface.

Checking interface implementation at compile time is possible by assigning a zero value of the interface to a variable of a specific type or using _var _ interface = (*Type)(nil). For example:

var _ io.Reader = (*MyType)(nil) // check that MyType implements io.Reader

If MyType does not implement all methods of io.Reader, the compiler will produce an error.