Erlang equivalent to lists:append(ListOfLists)?

BorjaEst · May 6, 2020, 6:25pm

Hello,

I have been looking for the equivalent of lists:append(ListOfLists) from Erlang in Elixir.

It terms of performance this is quite relevant as flatten is quite slower, see:
http://erlang.org/doc/efficiency_guide/listHandling.html#deep-and-flat-lists

BR, Borja

kokolegorille · July 22, 2020, 2:09pm

You can use Erlang functions in Elixir…

:list.append(list_of_list)

benwilson512 · May 6, 2020, 7:06pm

There is also Enum.concat which does basically the same thing.

BorjaEst · May 6, 2020, 7:20pm

Almost but not exactly the same:

iex(25)> x = for l <- 1..100, do: for b <- 1..100, do: :a
[:a, :a, ...]

iex(33)> :timer.tc(fn -> :lists.append(x) end)
{405, [:a, :a, ...]}

iex(34)> :timer.tc(fn -> Enum.concat(x) end)  
{1039, [:a, :a, ...]}

If we test it more than 1 time:

iex(51)> Enum.sum(for _ <- 1..100, do: elem(:timer.tc(fn -> Enum.concat(x) end),0))
21491
iex(52)> Enum.sum(for _ <- 1..100, do: elem(:timer.tc(fn -> :lists.append(x) end),0))
5881

NobbZ · May 6, 2020, 9:05pm

This is more or less expected. Enum.concat/1 is polymorphic, :lists.append/1 is specialised for lists.

Enum.concat/1 just calls Enum.reduce/3, which for lists shortcuts to Lists.foldl/3 IIRC:

github.com

elixir-lang/elixir/blob/v1.10.3/lib/elixir/lib/enum.ex#L555-L558


def concat(enumerables) do
  fun = &[&1 | &2]
  enumerables |> reduce([], &reduce(&1, &2, fun)) |> :lists.reverse()
end

:lists.append/1 though (I can’t find its source right now), might use :lists.foldr/3 as a matter of optimisation.