Now we have the missing piece we needed to explain the design of the append built-in function. The signature of append is different from our custom Append function above. Schematically, it’s like this:
func append(slice []T, elements …T) []T where T is a placeholder for any given type. You can’t actually write a function in Go where the type T is determined by the caller. That’s why append is built in: it needs support from the compiler.
What append does is append the elements to the end of the slice and return the result. The result needs to be returned because, as with our hand-written Append, the underlying array may change. This simple example
x := []int{1,2,3}
x = append(x, 4, 5, 6)
fmt.Println(x)
prints [1 2 3 4 5 6]. So append works a little like Printf, collecting an arbitrary number of arguments.
But what if we wanted to do what our Append does and append a slice to a slice? Easy: use … at the call site, just as we did in the call to Output above. This snippet produces identical output to the one above.
x := []int{1,2,3}
y := []int{4,5,6}
x = append(x, y...)
fmt.Println(x)
Without that …, it wouldn’t compile because the types would be wrong; y is not of type int.
origin from: https://golang.google.cn/doc/effective_go.html#append