Prolog ERROR: Undefined procedure, <= involved

Question

我正在编写 prolog 中的程序，遇到以下问题：我定义了谓词 neighbors(+,+,+,+,?) 如下：

neighbors(X, Y, Height, Width, Neighbors):-
   Xup is X-1,
   Xdown is X+1,
   Yleft is Y-1,
   Yright is Y+1,
   findall((A,B,C),(
            between(Xup, Xdown, A),
            between(Yleft, Yright, B),
            A>=1,
            B>=1),
            Neighbors).

现在查询 neighbors(5,5,5,5,X) 按预期工作，将 X 与其邻居列表统一，即

X = [ (4, 4, _G2809), (4, 5, _G2800), (4, 6, _G2791), (5, 4, _G2782), (5, 5, _G2773), (5, 6, _G2764), (6, 4, _G2755), (6, ..., ...), (..., ...)] .

但是，当我尝试将以下行添加到我的 findall 目标时出现问题：

A<=Height,
B<=Width

完整的谓词如下所示：

neighbors(X, Y, Height, Width, Neighbors):-
   Xup is X-1,
   Xdown is X+1,
   Yleft is Y-1,
   Yright is Y+1,
   findall((A,B,C),(
            between(Xup, Xdown, A),
            between(Yleft, Yright, B),
            A>=1,
            B>=1,
            A<=Height,
            B<=Width
            ),
            Neighbors).

现在相同的查询，neighbors(5,5,5,5,X)。导致我收到以下错误：

ERROR: Undefined procedure: neighbors/5
ERROR:     However, there are definitions for:
ERROR:         neighbor/2
ERROR:         neighbors/2
false.

可能是什么原因？我想这与我比较这些变量的方式有关，但由于 Width 和 Height 是实例化的，我认为这应该有效。谢谢。

Answer 1

问题出在你的比较运算符上。小于或等于运算符的语法是 =</2。所以你的目标应该是：

...
A=<Height,
B=<Width
...