% you may need to update the module directive that follows for
% compatibility with your Prolog compiler module system
:- module(module, [mod_length/2]).
mod_length(List, Length) :-
( integer(Length) ->
Length >= 0,
mod_make_list(Length, List)
; mod_length(List, 0, Length)
).
mod_make_list(0, []):-
!.
mod_make_list(N, [_| Tail]):-
M is N-1,
mod_make_list(M, Tail).
mod_length([], Length, Length).
mod_length([_| Tail], Acc, Length) :-
Acc2 is Acc + 1,
mod_length(Tail, Acc2, Length).
