Question

Situation:

I’m trying to write a simple fmt.Fprintf wrapper which takes a variable number of arguments. This is the code:

func Die(format string, args ...interface{}) {
    str := fmt.Sprintf(format, args)
    fmt.Fprintf(os.Stderr, "%v\n", str)
    os.Exit(1)
}

Problem:

When I call it with Die("foo"), I get the following output (instead of “foo“):

foo%!(EXTRA []interface {}=[])

• Why is there “%!(EXTRA []interface {}=[])” after the “foo“?
• What is the correct way to create wrappers around fmt.Fprintf?

Answer 1

Variadic functions receive the arguments as a slice of the type. In this case your function receives a []interface{} named args. When you pass that argument to fmt.Sprintf, you are passing it as a single argument of type []interface{}. What you really want is to pass each value in args as a separate argument (the same way you received them). To do this you must use the ... syntax.

str := fmt.Sprintf(format, args...)

This is also explained in the Go specification here.