adding to elixir lists

list.ex

# See https://hexdocs.pm/elixir/List.html for reference

iex> list = [1, 2, 3]

iex> [list | 4] # fast, but probably not what you want
[[1, 2, 3] | 4]

iex> [0 | list]   # fast
[0, 1, 2, 3]

iex> list ++ [4]  # slow
[1, 2, 3, 4]