List adding in Prolog

Question

If I have a list:

food_price(banana,2).
food_price(apple,1).
food_price(chocolate,5).
food_price(milk,3).
food_price(coffee,4).

And if I want to sum up the price like this using 'total_price/2':

?- total_price([banana, apple, milk], Price).
   Price = 6.

How can I get it work? I only know adding up number in a list in prolog can be written like this:

sum([H|T], Price) :- sum(T, H, Price).
sum([], Acc, Acc).
sum([H|T], Acc, Price) :- NewAcc is H + Acc, sum(T, NewAcc, Price).

But how can I match the price with the name of food? Thanks.

Answer 1

This can easily be done with maplist/3:

total_price(FoodList, TotalPrice) :-
    maplist(food_price, FoodList, PriceList),
    sumlist(PriceList, TotalPrice).

maplist/3 applies the food_price call to each element of FoodList and provides the corresponding success arguments in PriceList. Then you just add them up with sumlist/2.

If you don't want to use maplist or sumlist, you can create what these do easily with simple list recursion.