在Prolog中构建表达式树
问题描述：
我正在寻找一种在Prolog中构建表达式树的方法.我已经做过一些实验，并提出了以下工作代码(仅处理常量和加号表达式):
I'm looking for a way to build an Expression Tree in Prolog. I already did some experiments and came up with the following working code (that will only handle constants and the plus expression):
const(_).
plus(_, _).
eval(const(R), R).
eval(plus(A, B), R) :- number(A), number(B), R is A+B.
eval(plus(A, B), R) :- number(A), eval(B, B_R), R is A+B_R.
eval(plus(A, B), R) :- eval(A, A_R), number(B), R is A_R+B.
eval(plus(A, B), R) :- eval(A, A_R), eval(B, B_R), R is A_R+B_R.
除了这种方法，还有其他更简单的选择吗?我是否需要为计划添加到程序中的每个运算符定义这4种情况?
Is there any simpler alternative to this approach? Will I have to define these 4 cases for each one of the operators I plan on adding to my program?
答
我认为应该这样做，尽管我对结构pred1(pred2(...)...) :- ...并不熟悉(我的Prolog非常生锈).
I think this should do it, though I'm not familiar with the construct pred1(pred2(...)...) :- ... (my Prolog is very rusty).
eval(A, A) :- number(A).
eval(plus(A, B), R) :- eval(A, A_R), eval(B, B_R), R is A_R+B_R.