N-Puzzle solver

n_puzzle.pl

ops(+).
ops(-).
ops(*).
ops(/).

calc([], [Result], Result).
calc([+ | Rest], [B, A | Stack], Result) :- calc(Rest, [A + B | Stack], Result).
calc([- | Rest], [B, A | Stack], Result) :- calc(Rest, [A - B | Stack], Result).
calc([- | Rest], [B, A | Stack], Result) :- calc(Rest, [B - A | Stack], Result).
calc([* | Rest], [B, A | Stack], Result) :- calc(Rest, [A * B | Stack], Result).
calc([/ | Rest], [B, A | Stack], Result) :- B > 0, calc(Rest, [A / B | Stack], Result).
calc([/ | Rest], [B, A | Stack], Result) :- A > 0, calc(Rest, [B / A | Stack], Result).
calc([N | Rest], Stack, Result) :- number(N), calc(Rest, [N | Stack], Result).

expr([N], 0, 0, Expr, [N | Expr]).
expr(Stack, Nums, Ops, Expr, Result) :-
    Ops > 0,
    ops(Op),
    NextOps is Ops - 1,
    expr(Stack, Nums, NextOps, [Op | Expr], Result).
expr([N , M | Stack], Nums, Ops, Expr, Result) :-
    Nums > Ops,
    NextNums is Nums - 1,
    expr([M | Stack], NextNums, Ops, [N | Expr], Result).

solve_n_puzzle(N, Nums, Result) :-
    length(Nums, Len),
    Len > 0,
    Size is Len - 1,
    expr(Nums, Size, Size, [], Expr),
    calc(Expr, [], Result),
    Result =:= N.