>Go’s lack of generics means that you have to write mapStringToString as a mapper between collections of two fixed types – you can’t write a generic map from an array of T to an array of R, as you can in almost any other strongly-typed modern language.

That's not true. Here's their example written to be more generic:

  func mapThingToThing(inSlice interface{}, mapperFn interface{}) []interface{} {
  	mapper := reflect.ValueOf(mapperFn)
  	input := reflect.ValueOf(inSlice)
  	output := make([]interface{}, input.Len())
  	for i := 0; i < input.Len(); i++ {
  		item := input.Index(i)
  		output[i] = mapper.Call([]reflect.Value{item})[0]
  	}
  	return output
  }

and calling:

  fn := func(item string) string {
  	return item + "!"
  }
  res := mapThingToThing([]string{"a", "b", "c"}, fn)
  for _, item := range res {
  	fmt.Printf("item = %+v\n", item)
  }

I didn't say it was pretty or good, but it can be done. Really confused by the downvotes.