The example could have been simplified:

  func abs(x Top) Top {
	switch v := x.(type) {
	case int32:
		if v < 0 {
			return -v
		}
	case int64:
		if v < 0 {
			return -v
		}
	case float32:
		if v < 0 {
			return -v
		}
	case float64:
		if v < 0 {
			return -v
		}
	}
	return v
  }