Prolog: recursion is not working properly-CodePudding

I want to write change_state( L1, L2, -L3).

L1 contains states e.g: [milled, burned] L2 contains predicates e.g: [del(milled), add(shifted)] And L3 is the return list.

Example:

L1 = [milled, burned]
L2 = [del(burned), add(shifted)]
L3 = [milled, shifted]

So L1 is my start list and depending on the predicates in L2 I change the state of L1.

This is my solution with recursion:

change_state([], [], []).

change_state(X, [], []).

change_state(State, [H|T], NewState) :-
    functor(H, N, 1),
    arg(1, H, A),
    (    N = del 
    ->   remove_from_list(A, State, NewState)
    ;    arg(1, H, A),
         add_to_list(A, State, NewState) ),
    print(NewState),
    change_state(NewState, T, X),
    print(NewState).

I also put some prints into the function for debugging.

Now the problem:

If I call the function with

change_state([milled, burned], [del(burned), add(lol), add(hi)], L).

I get as an result

L = [milled].

So only the first predicate in L2 worked.

But the prints show me:

[milled][lol,milled][hi,lol,milled][hi,lol,milled][lol,milled][milled].

So it actually worked, but because of the recursion it goes back to the first state.

Any idea how to fix this?

CodePudding user response：

Although your code is already working correctly with the modifications that were indicated in the comments, I think a clearer implementation would be as follows:

change_state(State, Changes, NewState) :-
    change_state_loop(Changes, State, NewState).

% Reverse the order of the first two arguments to avoid choice points.

change_state_loop([], State, State).
change_state_loop([Change|Changes], State, NewState) :-
    do(Change, State, PartiallyUpdatedState),
    change_state_loop(Changes, PartiallyUpdatedState, NewState).

% Use unification to choose the correct operation to perform.

do(del(Fluent), State, NewState) :- subtract(State, [Fluent], NewState).
do(add(Fluent), State, NewState) :- union(State, [Fluent], NewState).