Tell me about interfaces in Go: what are they, what do they consist of, what is a nil interface?

In Go, an interface is a set of methods that a type must implement to be considered as implementing that interface. An interface describes behavior, not data structure.

An interface consists of a list of method signatures, for example:

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

Any type that implements the method Read([]byte) (int, error) automatically satisfies the Reader interface.

A nil interface is an interface that contains neither a value nor a type. Its value is nil. It is important to understand that in Go, an interface is a pair: (type, value). If both type and value are nil, the interface is considered nil. If the type is present but the value is nil, the interface is not nil.

Example:

var r io.Reader // r == nil

var p *os.File = nil
r = p // r != nil, because the type is *os.File, even though the value is nil

if r == nil {
    fmt.Println("r is nil")
} else {
    fmt.Println("r is not nil")
}

This often causes confusion when checking for nil with interfaces.