How to Prepend a String to a Variadic Interface{} Slice in Go?

Prepending a String to a Variable Argument Slice in Go

In Go, the append() function allows you to append elements to a slice of the same type. However, when dealing with a method that accepts a variable argument slice of type ...interface{}, appending a string directly using append("some string", v) results in an error.

To successfully prepend a string to the variable argument slice, you need to use a slice of interface{} values. This is because ...interface{} allows for elements of any type. To achieve this, you can wrap the initial string in a []interface{} slice:

func (l Log) Error(v ...interface{}) {
  stringInterface := []interface{}{" ERROR "}
  l.Out.Println(append(stringInterface, v...))
}

This wraps the ERROR string in a []interface{} slice, which can then be appended to the variable argument slice v.

Here's an example:

package main

import "fmt"

func main() {
  s := "first"
  rest := []interface{}{"second", 3}

  all := append([]interface{}{s}, rest...)
  fmt.Println(all)
}

Output:

[first second 3]

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