序言问题..无法调用谓词

Question

我试图理解为什么不调用 duck2。我是否应该涵盖两个列表为空、非空的所有情况。


duck([H|T], something ) :- 

    write("*"),
    write("?"),
    duck2([H|T], T, something_else),
    write("Over").

duck2([], [], something_else) :- write("AllDone").

duck2([H|T], [H1,T1], something_else) :-
    write("I am here").

trace 给出了这个..

Call: (16) duck([dog, eats], [eats], something) ? creep
   Call: (17) write("*") ? creep
*
   Exit: (17) write("*") ? creep
   Call: (17) write("?") ? creep
?
   Exit: (17) write("?") ? creep
   Call: (17) duck2([dog, eats], [eats], something_else) ? creep
   Fail: (17) duck2([dog, eats], [eats], something_else) ? creep
   Fail: (16) duck([dog, eats], [eats], something) ? creep

Answer 1

duck2([H|T], [H1,T1], something_else) :-

% spaced out to line up with the below trace
            duck2([H|T],       [H1,T1], something_else) :-
Call: (17) `duck2([dog, eats], [eats],  something_else)` ? creep

[H1, T1] 是两个项目的列表，[eats] 是一个项目的列表；如果您尝试统一它们，它们不会统一：

?- [H1, T1] = [eats].
false

所以调用失败。

Should I cover all cases

好吧，至少涵盖了足够多的案例以使代码正常工作。所有情况由您决定。

序言问题..无法调用谓词

prolog questions.. fails to invoke predicate

prolog