Why does `fmt.Sprintf` produce unexpected output when using a variadic parameter in a wrapper function?

Unraveling Variadic Parameters in Wrapper Functions

Variadic functions in Go play a crucial role in accepting an arbitrary number of arguments. To utilize this feature effectively, it's essential to understand how these parameters are handled.

Consider the example of a fmt.Fprintf wrapper function:

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

When invoking this function with Die("foo"), a rather unexpected output emerges: foo%!(EXTRA []interface {}=[]). This perplexing suffix can be attributed to the way variadic parameters are passed.

By design, variadic functions receive arguments as a slice of their respective type. In this case, the Die function accepts a []interface{} slice named args. However, when subsequently passing this argument to fmt.Sprintf, it's treated as a single entity of type []interface{}, rather than the intended individual values.

To rectify this, the ... syntax should be employed as seen below:

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

Using this approach ensures that each value within args is passed to fmt.Sprintf as a separate argument, mimicking the behavior when receiving them in the Die function. This crucial distinction aligns with the Go language specification and guarantees the expected output.

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