[m-rev.] for review: replace_nth_element[01]

[Julien Fischer]

On Sat, 15 Aug 2026 at 10:52, Zoltan Somogyi <zoltan.somogyi at runbox.com> wrote:

>
> Unless you object, I intend to add the same predicates
> to one_or_more.m as well, in a separate diff.

No objection..

> Move {det_,}replace_nth_elemente[01] to list.m.
>
> library/list.m:
>     As above.
>
> NEWS.md:
>     Announce the additions to list.m.
>
> compiler/input_specialization.m:
>     Delete the moved code.

...

> diff --git a/library/list.m b/library/list.m
> index 8ccec97f5..703802c5e 100644
> --- a/library/list.m
> +++ b/library/list.m
> @@ -429,6 +429,12 @@
>      % Fails if N < 1 or if length of List0 < N.
>      % (Position numbers start from 1.)
>      %
> +    % NOTE: please consider using one of replace_nth_element[01] below,
> +    % since these
> +    %
> +    % - clearly specify how they number the list's elements, and
> +    % - their argument order is suitable for the use of state variables.
> +    %
>  :- pred replace_nth(list(T)::in, int::in, T::in, list(T)::out) is semidet.
>
>      % det_replace_nth(List0, N, R) = List:
> @@ -438,9 +444,49 @@
>      % Throw an exception if either N < 1, or if length of List0 < N.
>      % (Position numbers start from 1.)
>      %
> +    % NOTE: please consider using one of det_replace_nth_element[01] below,
> +    % since these
> +    %
> +    % - clearly specify how they number the list's elements, and
> +    % - their argument order is suitable for the use of state variables.
> +    %
>  :- func det_replace_nth(list(T), int, T) = list(T).
>  :- pred det_replace_nth(list(T)::in, int::in, T::in, list(T)::out) is det.
>
> +    % replace_nth_element1(N, R, List0, List):
> +    % det_replace_nth_element1(N, R, List0, List):
> +    %
> +    % Succeed if-and-only-if List is List0 with its element #N
> +    % replaced with R. (The first element is element #1.)
> +    %
> +    % If List0 has no element #N, either N < 1, or because
> +    % the length of List0 is less than N, then
> +    %
> +    % - replace_nth_element0 fails, while
> +    % - det_replace_nth_element0 fails throws an exception.

Delete the word "fails" in the second dot point.

> +:- pred replace_nth_element1(int::in, T::in, list(T)::in, list(T)::out)
> +    is semidet.
> +:- pred det_replace_nth_element1(int::in, T::in, list(T)::in, list(T)::out)
> +    is det.
> +
> +    % replace_nth_element0(N, R, List0, List):
> +    % det_replace_nth_element0(N, R, List0, List):
> +    %
> +    % Succeed if-and-only-if List is List0 with its element #N
> +    % replaced with R. (The first element is element #0.)
> +    %
> +    % If List0 has no element #N, either N < 0, or because
> +    % the length of List0 is less than N-1, then
> +    %
> +    % - replace_nth_element0 fails, while
> +    % - det_replace_nth_element0 fails throws an exception.

And again there.

The rest looks fine.

Julien.