[m-users.] Why does list.length use an auxiliary function?

[Jeffrey Brown]

The length function from the list module[1] is a lot of code. This simple
definition seems sufficient:

    :- pred simple_length(list(_T), int).
    :- mode simple_length(in, out) is det.
    :- func simple_length(list(T)) = int.

    simple_length( [], 0 ).
    simple_length( [_|Rest], N ) :-
      simple_length( Rest, N0 ),
      N = 1 + N0.
    simple_length(List) = N :-
      simple_length(List, N).

Why is it instead this more complex one?

    :- pred length(list(_T), int).
    :- mode length(in, out) is det.
    :- func length(list(T)) = int.

    length(L, N) :-
        list.length_acc(L, 0, N).

    length(Xs) = N :-
        list.length(Xs, N).

    :- pred length_acc(list(T), int, int).
    :- mode length_acc(in, in, out) is det.

    length_acc([], N, N).
    length_acc([_ | L1], N0, N) :-
        N1 = N0 + 1,
        list.length_acc(L1, N1, N).

[1] https://github.com/Mercury-Language/mercury/blob/master/library/list.m


-- 
Jeff Brown | Jeffrey Benjamin Brown
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