append_vs_prepend.exs

defmodule ConcatTest do
  def append do
    1..10000 |> Enum.reduce([], fn (x, acc) ->
      acc ++ [x]
    end)
  end
  def prepend do
    1..10000 |> Enum.reduce([], fn (x, acc) ->
      [x] ++ acc
    end) |> Enum.reverse
  end
end

{time, _} = :timer.tc fn -> ConcatTest.append end
IO.puts "Append took #{time}"

{time, _} = :timer.tc fn -> ConcatTest.prepend end
IO.puts "Prepend took #{time}"

➜  /tmp  elixir append_vs_prepend.exs
Append took 310895
Prepend took 2399

defmodule ConcatTest do
  def append do
    1..100000 |> Enum.reduce([], fn (x, acc) ->
      acc ++ [x]
    end)
  end
  def prepend do
    1..100000 |> Enum.reduce([], fn (x, acc) ->
      [x] ++ acc
    end) |> Enum.reverse
  end
  def concat do
    1..100000 |> Enum.reduce([], fn (x, acc) ->
      [x | acc]
    end) |> Enum.reverse
  end
  def run do
    {time, _} = :timer.tc fn -> ConcatTest.append end
    IO.puts "Append took #{time}"

    {time, _} = :timer.tc fn -> ConcatTest.prepend end
    IO.puts "Prepend took #{time}"

    {time, _} = :timer.tc fn -> ConcatTest.concat end
    IO.puts "concat took #{time}"
  end
end

iex(4)> ConcatTest.run
Append took 23567395
Prepend took 8995
concat took 7881

interestingly, concat [ a | b] is consistently 10% faster for me. I'd also be interested in memory usage.