递归查询，检查X是否存在于Prolog数据库中。

Question

我试图创建一个查询 exists(X) 如果X存在于prolog数据库中，返回true。

Prolog数据库

store(best_smoothies, [alan,john,mary],
      [ smoothie(berry, [orange, blueberry, strawberry], 2),
        smoothie(tropical, [orange, banana, mango, guava], 3),
        smoothie(blue, [banana, blueberry], 3) ]).
store(all_smoothies, [keith,mary],
      [ smoothie(pinacolada, [orange, pineapple, coconut], 2),
        smoothie(green, [orange, banana, kiwi], 5),
        smoothie(purple, [orange, blueberry, strawberry], 2),
        smoothie(smooth, [orange, banana, mango],1) ]).
store(smoothies_galore, [heath,john,michelle],
      [ smoothie(combo1, [strawberry, orange, banana], 2),
        smoothie(combo2, [banana, orange], 5),
        smoothie(combo3, [orange, peach, banana], 2),
        smoothie(combo4, [guava, mango, papaya, orange],1),
        smoothie(combo5, [grapefruit, banana, pear],1) ]).

我的尝试

exists(X) :- store(_,_,S), isIn(X,S).

isIn(X, [smoothie(X,_,_)|[]]).
isIn(X, [smoothie(N,_,_)|T]) :- isIn(X,T).

令人惊讶的是，它只对 blue, smooth 和 combo5. 是不是我的递归有问题？(我认为没有，因为它能够读出 blue, smooth 一直到 combo5)

我得到的输出

?- exists(combo1).
false

?- exists(X).
X = blue;
X = smooth;
X = combo5;
false

感谢任何帮助。非常感谢!

编辑：修改后的尝试

exists(X) :- store(_,_,S), isIn(X,S).

isIn(X, [smoothie(X,_,_)]).
isIn(X, [smoothie(N,_,_)|T]) :- isIn(X,T).

EDIT: 修正

exists(X) :- store(_,_,S), isIn(X,S).

isIn(X, [smoothie(X,_,_)|_]).
isIn(X, [smoothie(N,_,_)|T]) :- isIn(X,T).

Answer 1

[smoothie(X,_,_) | []] 与 [smoothie(X,_,_)].

isIn( X, S ) 相当于

isIn( X, S ) :-
  last(  S, smoothie(X,_,_) ).

这解释了你的观察。