building a list with Recursion with Prolog - recursion

Question

I have a recursive predicate that builds a list like it's suposed to, the problem is that eventually it starts 'debuilding it' and gives me an empty list, the linha predicate is ok, it gives a value to Vel, and Or.
actualizarveiculos(_,[],D).
actualizarveiculos(Tempo,[(X,Y)|RestoVeiculos],NovaVeiculos):-
linha(Y,Or,Vel),
faz_andar_veic(Or,(X,Y),C,mod(Tempo,Vel)),
append(NovaVeiculos,[C],D),
actualizarveiculos(Tempo,RestoVeiculos,D).
faz_andar_veic(90,(X,Y),(X+1,Y),0).
faz_andar_veic(270,(X,Y),(X-1,Y),0).
faz_andar_veic(_,(X,Y),(X,Y),C):-
C=\=0.
the trace

Answer 1

You need to change it as follows:
actualizarveiculos(_,[],D, D).
actualizarveiculos(Tempo,[(X,Y)|RestoVeiculos],Acc, Res):-
Or is 2,
Vel is 2,
faz_andar_veic(Or,(X,Y),C,mod(Tempo,Vel)),
append(Acc,[C],D),
actualizarveiculos(Tempo,RestoVeiculos,D, Res).
faz_andar_veic(90,(X,Y),(X+1,Y),0).
faz_andar_veic(270,(X,Y),(X-1,Y),0).
faz_andar_veic(_,(X,Y),(X,Y),C):-
C=\=0.
Test run:
[debug] ?- actualizarveiculos(5, [(-5,3), (-4,4), (-4,5)], [], Res).
Res = [ (-5, 3), (-4, 4), (-4, 5)] .
In the trace you provided you query actualizarveiculos(5, [(-5,3), (-4,4), (-4,5)], N). Since N is unbound then NovaVeiculos becomes unbound and then you do: append(NovaVeiculos,[C],D), which probably does not do what you think it does or atleast what you intended it to do. What it does is that it tries to append an unbound variable with a list and the result is unified with another unbound variable which obviously can give an infinite number of solutions, e.g.:
[debug] ?- actualizarveiculos(5, [(-5,3), (-4,4), (-4,5)], N).
N = [] ;
N = [_G3430] ;
N = [_G3430, _G3436] ;
N = [_G3430, _G3436, _G3442] ;
N = [_G3430, _G3436, _G3442, _G3448] ;
N = [_G3430, _G3436, _G3442, _G3448, _G3454] ;
N = [_G3430, _G3436, _G3442, _G3448, _G3454, _G3460] ;
N = [_G3430, _G3436, _G3442, _G3448, _G3454, _G3460, _G3466] ;
N = [_G3430, _G3436, _G3442, _G3448, _G3454, _G3460, _G3466, _G3472] ;
N = [_G3430, _G3436, _G3442, _G3448, _G3454, _G3460, _G3466, _G3472, _G3478] ;
N = [_G3430, _G3436, _G3442, _G3448, _G3454, _G3460, _G3466, _G3472, _G3478|...] ;
Hope it helps, comment if you have questions. Since you didn't provide the linha/3 predicate I just exchanged it with two unifications:
Or is 2,
Vel is 2,