List.init violates evaluation order of standard library

[mmottl]

The following code demonstrates inconsistent behavior between the List.init function in Base and the same function in the OCaml standard library due to an incorrect evaluation order of list elements:

open Printf
open Base

let () =
  printf "Stdlib:\n\n";
  let r = ref 0 in
  Stdlib.List.init 10 (fun _i -> Int.incr r; !r) |>
  List.iter ~f:(printf "%d\n")

let () =
  printf "\n\nJane Street:\n\n";
  let r = ref 0 in
  List.init 10 ~f:(fun _i -> Int.incr r; !r) |>
  List.iter ~f:(printf "%d\n")

Output:

Stdlib:

1
2
3
4
5
6
7
8
9
10


Jane Street:

10
9
8
7
6
5
4
3
2
1

Note that the standard library uses [@tail_mod_cons] to achieve tail recursion without having to reverse the list.

[nmatschke]

Thanks for the report. We will consider changing this, though there's some concern about code implicitly relying on the current evaluation order.