Credence.Pattern.NoListLast (credence v0.4.0)

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Performance rule: Flags usage of List.last/1.

Elixir lists are linked lists — accessing the last element requires traversing the entire list, making List.last/1 an O(n) operation. This is often a sign that the algorithm should be restructured to avoid needing the last element entirely.

Common refactors

  • If building a list with Enum.reduce, track the last value in the accumulator instead of extracting it afterward
  • If splitting a list, destructure the right half instead of taking the last of the left half
  • If the list was sorted or built in a known order, consider whether hd/1 on a reversed or desc-sorted list gives you the answer

Bad

Enum.reduce(1..(rows - 1), initial_row, fn _, prev ->
  Enum.scan(prev, &(&1 + &2))
end)
|> List.last()

Good — track the answer in the accumulator

{_row, last} =
  Enum.reduce(1..(rows - 1), {initial_row, 1}, fn _, {prev, _} ->
    row = Enum.scan(prev, &(&1 + &2))
    {row, List.last(row)}  # or track running total differently
  end)