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

[Zoltan Somogyi]

On Sun, 23 Dec 2018 01:10:36 -0500, Jeffrey Brown <jeffbrown.the at gmail.com> wrote:
>     simple_length( [], 0 ).
>     simple_length( [_|Rest], N ) :-
>       simple_length( Rest, N0 ),
>       N = 1 + N0.
> 
> Why is it instead this more complex one?
>
>     length_acc([], N, N).
>     length_acc([_ | L1], N0, N) :-
>         N1 = N0 + 1,
>         list.length_acc(L1, N1, N).

Because length_acc is tail recursive (i.e. the recursive clause does nothing
after the recursive call, while simple_length is NOT tail recursive (since
it increments the result of the recursive call). This makes the accumulator version
more efficient. See the wikipedia page on tail recursion.

Zoltan.