If I understood correctly you want to return the %x format:
you can import hex and use the EncodeToString
method
str := hex.EncodeToString(h.Sum(nil))
or just Sprintf the value:
func md(str string) string {
h := md5.New()
io.WriteString(h, str)
return fmt.Sprintf("%x", h.Sum(nil))
}
note that Sprintf is slower because it needs to parse the format string and then reflect based on the type found