let rec f evens odds = function
    | [] -> (evens, odds)
    | x :: xs  ->
      if x mod 2 = 0 then f (x :: evens) odds xs
      else f evens (x :: odds) xs

However OCaml also provides the "tail mod cons" optimization that allows to get some of the benefits of TCO without needing to reverse the list (this is implemented as a program transformation that uses mutability under the hood), and that one will only work if you are building a single list.

The OCaml compiler disagrees with you ;)

It also won't ever turn a cons operation into a dynamic array append.

But sure, in the `fold` scenario where you don't know the number of results in advance (you are more likely to know if you use imperative data structures, e.g. `Hashtbl.length` is constant-time whereas `Map.cardinal` is not), lists might be faster than growing arrays with copies. They are still going to use more memory, and they are unlikely to to be faster than a rope-like structure that grows with no copies.

A parallel `Array.map` still computes a sequence, even though it may not compute in sequence.