% 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).