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W0056 - Use lists:reverse/2 instead of lists:reverse/1 ++ Tail

Warning​

reverse_and_append(List, Tail) ->
    lists:reverse(List) ++ Tail.
%%  ^^^^^^^^^^^^^^^^^^^^^^^^^^^ 💡 warning: W0056: Use `lists:reverse/2` instead of `lists:reverse/1 ++ Tail` for better performance.

Explanation​

The warning indicates that you are using the pattern lists:reverse(List) ++ Tail, which is inefficient. This pattern first reverses the list, creating an intermediate reversed list, and then appends the tail to it using the ++ operator.

The Erlang standard library provides a more efficient function lists:reverse/2 that performs both operations in a single pass without creating the intermediate reversed list.

Why This Matters​

  • Performance: Using lists:reverse/1 followed by ++ creates an unnecessary intermediate data structure
  • Memory: The intermediate reversed list consumes additional memory that is immediately discarded
  • Idiomatic Erlang: lists:reverse/2 is the idiomatic way to reverse and prepend in Erlang

Fix​

Replace the pattern with lists:reverse/2:

reverse_and_append(List, Tail) ->
    lists:reverse(List, Tail).

How lists:reverse/2 Works​

The second argument to lists:reverse/2 acts as an accumulator that gets prepended to the reversed list:

lists:reverse([1, 2, 3], [4, 5, 6]).
% Returns: [3, 2, 1, 4, 5, 6]

This is equivalent to lists:reverse([1, 2, 3]) ++ [4, 5, 6] but significantly more efficient.