golang prepend a string to a slice ...interface{}

Question

I've a method that has as an argument v ...interface{}, I need to prepend this slice with a string. Here is the method:

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

When I try with append() it doesn't work:

> append("some string", v)
first argument to append must be slice; have untyped string
> append([]string{"some string"}, v)
cannot use v (type []interface {}) as type string in append

What's the proper way to prepend in this case?

Answer 1

append() can only append values of the type matching the element type of the slice:

func append(slice []Type, elems ...Type) []Type

So if you have the elements as []interface{}, you have to wrap your initial string in a []interface{} to be able to use append():

s := "first"
rest := []interface{}{"second", 3}

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

Output (try it on the Go Playground):

[first second 3]