小编Tom*_*ath的帖子

这是递归的一个简单序言示例。我不知道在哪里，或多或少地如何声明退出语句。试飞(sofia, dublin) 应该返回true，但它会在最后一步不断检查你是否可以directFlight(dublin, dublin)。这是代码：

directFlight(sofia, varna).
directFlight(sofia, paris).
directFlight(sofia, london).
directFlight(london, edinburg).
directFlight(paris, new_york).
directFlight(new_york, seattle).
directFlight(london, dublin).

flight(City1, City2) :- 
   directFlight(City1, City3),      
   flight(City3, City2).

输出：

 [trace]  ?- flight(sofia, dublin).
   Call: (8) flight(sofia, dublin) ? creep
   Call: (9) directFlight(sofia, _878) ? creep
   Exit: (9) directFlight(sofia, varna) ? creep
   Call: (9) flight(varna, dublin) ? creep
   Call: (10) directFlight(varna, _878) ? creep
   Fail: (10) directFlight(varna, _878) ? creep
   Fail: (9) flight(varna, dublin) ? creep
   Redo: (9) directFlight(sofia, _878) ? creep
   Exit: (9) …