import (
	"fmt"
	"strings"
)

func foo(x string) string {
	return strings.NewReplacer("?", "!").Replace(x)
}

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

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

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