A Tour of Go: Exercise: Readers, https://go-tour-jp.appspot.com/methods/22

exercise-reader.go

package main

import "golang.org/x/tour/reader"

type MyReader struct{}

// Add a Read([]byte) (int, error) method to MyReader.
func (r MyReader) Read(b []byte) (int, error) {
	b[0] = 'A'
	return 1, nil
}

func main() {
	reader.Validate(MyReader{})
}

see also: https://github.com/golang/tour/blob/master/reader/validate.go