Farmer, Wolf, Goat and Cabbage
A classic problem from Artificial Intelligence: Structures and strategies for complex problem solving by George F. Luger and William A. Stubblefield.
- A farmer wishes to transfer (by boat) a wolf, a goat, and a cabbage from the left bank of a river to the right bank. If left unsupervised, the wolf will eat the goat and the goat will eat the cabbage, but nothing will happen as long as the farmer is near.
- Beside the farmer there is only room for one item in the boat.
This is a way to solve the problem in Visual Prolog:
implement main domains bank = start; target. state = s(bank Farmer, bank Wolf, bank Goat, bank Cabbage). constants initState : state = s(start, start, start, start). goalState : state = s(target, target, target, target). domains cargo = alone; wolf; goat; cabbage. move = m(cargo Cargo, bank After). class predicates solve : (state InitState) -> move* MoveList determ. clauses solve(Init) = MoveList :- MoveList = solve1(Init, [Init]), !. class predicates solve1 : (state S1, state* PreviousStates) -> move* MoveList nondeterm. clauses solve1(goalState, _PreviousStates) =  :- % solved !. solve1(S1, PreviousStates) = [Move | MoveList] :- % make one step Move = move_nd(S1, S2), % avoid illegal states not(illegalState(S2)), % avoid states that we have already been in not(list::isMember(S2, PreviousStates)), % solve the rest MoveList = solve1(S2, [S2|PreviousStates]). class predicates move_nd : (state Before, state After [out]) -> move Move multi. clauses move_nd(s(A, X, Y, Z), s(B, X, Y, Z)) = m(alone, B) :- % sail alone B = oppositeBank(A). move_nd(s(A, A, X, Y), s(B, B, X, Y)) = m(wolf, B) :- % sail with wolf B = oppositeBank(A). move_nd(s(A, X, A, Y), s(B, X, B, Y)) = m(goat, B) :- % sail with goat B = oppositeBank(A). move_nd(s(A, X, Y, A), s(B, X, Y, B)) = m(cabbage, B) :- % sail with cabbage B = oppositeBank(A). class predicates oppositeBank : (bank Bank) -> bank Opposite. clauses oppositeBank(start) = target. oppositeBank(target) = start. class predicates illegalState : (state State) determ. clauses illegalState(s(A, B, B, _)) :- % wolf and goat opposite farmer B = oppositeBank(A), !. illegalState(s(A, _, B, B)) :- % goat and cabbage opposite farmer B = oppositeBank(A). clauses run():- console::init(), if MoveList = solve(initState) then foreach m(A,B) = list::getMember_nd(MoveList) do stdio::writef("Sail % to %\n", A, B) end foreach else stdio::write("No solution") end if. end implement main