import "fmt"

var x = "Hello, world!"

foo := func(prompt string) (n int, err error) {
	n, err = fmt.Println(prompt + x)
	return
}

fmt.Println(foo("x: "))

printf := func(format string, args ...interface{}) (n int, err error) {
	n, err = fmt.Printf(format, args...)
	return
}

bar := func(foo func(string, ...interface{}) (int, error)) {
	foo("Hello, %v!\n", "Go+")
}

bar(printf)
fmt.Println(printf("Hello, %v\n", "Go+"))