What is an interface in Go and how is it structured internally?

In Go, an interface is a set of methods that describe behavior. Any type that implements all the methods of the interface automatically satisfies that interface, without explicit declaration.

Inside, the interface is represented by a structure with two fields: a pointer to the type descriptor and a pointer to the data (value). This allows the interface to hold a value of any type that implements the interface's methods and to call them dynamically.

Example of an interface and its implementation:

type Reader interface {
    Read(p []byte) (n int, err error)
}

type File struct {}

func (f *File) Read(p []byte) (int, error) {
    // implementation of read
    return 0, nil
}

var r Reader = &File{} // File implements Reader

Thus, in Go, an interface is a polymorphism mechanism based on implicit implementation and dynamic binding through internal pointers.