prolog - Split a list in half

Question

I need to define divide so that List [1,2,3,4,5] divides into:

a = [1,2,3}

b = [4,5]

I'm getting an error that says "Arguments are not sufficiently instantiated", and I don't know enough about the language to figure out what my problem is, or if my design is even right. Any guidance would be appreciated.

So here's what I have so far:

append([],L2,L2).
append([H|T],L2,[H|L3]) :- append(T,L2,L3).

lengthIs([],N).
lengthIs([H|T],N) :- lengthIs(T,M), N is M+1.

divide([],[],[]).
divide([H|T],L2,L3) :-
   (  lengthIs(L2, M) < lengthIs(L1,N)/2
   -> divide(T, append(L2, H, X), L3)
   ;  divide(T, L2, append(L3,H,Y))
   ).

Answer 1

Let's give the predicate a more relational name: list_half_half/3

list_half_half(Xs, Ys, Zs) :-
   length(Xs, N),
   H is N - N // 2,
   length(Ys, H),
   append(Ys, Zs, Xs).

length/2 and append/3 are predefined in practically all recent Prologs.

This is GNU Prolog:

| ?- append(L,_,[a,b,c,d]), list_half_half(L,H1,H2).

H1 = []
H2 = []
L = [] ? ;

H1 = [a]
H2 = []
L = [a] ? ;

H1 = [a]
H2 = [b]
L = [a,b] ? ;

H1 = [a,b]
H2 = [c]
L = [a,b,c] ? ;

H1 = [a,b]
H2 = [c,d]
L = [a,b,c,d]