how to explain this program to implement interface

https://play.golang.org/p/LHkVGzmC7N

look this source.

specilly this scrap:

bw := NewWriter(b)
w, ok := bw.wr.(io.ReaderFrom)

i dont understand b is bytes element，NewWrite() take a io.Writer。
and bw.wr.(io.ReaderFrom),how use is?

what's mean the ".(io.ReaderFrom)" 's function?

and

   fmt.Println(w.ReadFrom(s))

w is io.write,in io/io.go the ReadFrom(s) is interface.

type ReaderFrom interface {
    ReadFrom(r Reader) (n int64, err error)
}

how in this source can implement this interface?

in this source ,i cant find anywhere to implement.

It is a type assertion.

In your case it asserts that w is not nil and that the value stored in w is of interface io.ReaderFrom. ok is going to be true if it is, and false otherwise. This code doest not check ok variable because of the author's confidence it will be implementing io.ReaderFrom interface.

• bytes.Buffer implements func (b *Buffer) Write(p []byte) (n int, err error), so it is of type io.Writer and can serve as parameter to func NewWriter(w io.Writer) *Writer

• bytes.Buffer also implements func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error), so it is of type io.ReadFrom, which enables the call for fmt.Println(w.ReadFrom(s))

• as @akond mentioned .(io.ReaderFrom) is type assertion, and the expression w, ok := bw.wr.(io.ReaderFrom) asserts that the wr field of the Writer struct is also of type io.ReaderFrom

For further reading check laws-of-reflection, it refers to similar code.