using union to merge 2 interfaces

interface-union.go

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

type Writer interface {
	Write(p []byte) (n int, err error)
	Close() error
}

// ReadWriter's methods are Read, Write, and Close.
type ReadWriter interface {
	Reader // includes methods of Reader in ReadWriter's method set
	Writer // includes methods of Writer in ReadWriter's method set
	// Close()
}