Prolog 中递归后返回一个值
我决定学习一些逻辑编程，然后我偶然发现了一个问题。 它是在 SWI Prolog 中编程的。
test(A, B, N):-
nonvar(B),
B = final,
true.
test(A, B, N):-
N > 2,
test(A, final, N).
test(A, B, N):-
N1 is N + 1,
test(N1, B, N1).
这只是一个样本，没有实际用途，只是它让我发疯。
问题是，当代码达到 true 时，它就会开始回溯并回答“true”。但我需要“返回”值A。我该怎么做？
I decided to study some logic programming and I stumbled across a problem.
It is programmed in SWI Prolog.
test(A, B, N):-
nonvar(B),
B = final,
true.
test(A, B, N):-
N > 2,
test(A, final, N).
test(A, B, N):-
N1 is N + 1,
test(N1, B, N1).
It is just a sample with no real use except it is driving me crazy.
The problem is that when the code reaches true then it starts tracking back and answers "true". But I need to "return" value A. How do I do that?
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A 没有与规则正文中的任何内容统一。 prolog 的工作方式是通过术语的统一。您不能像过程语言中那样“返回”A。例如，当递归结束时，你希望 A 的值是多少？我不知道你的代码在做什么，所以让我使用我自己的例子。
下面是一个 sum 过程，它将对列表中的值求和并“返回 N”，即列表中的值的总和。要调用此过程，您可以这样做：
Prolog 将响应：
请注意，随着递归的进行，accumulate 过程使用 A 作为累加器。也就是说，A 保留运行总和，而 N 是它返回的最终答案。在递归过程中，N 不与任何实际值统一。
在递归的最后一步，即列表为空时，A的值与N统一，实际上返回N。
让我们做一个Trace。
现在，请注意跟踪中我标记
//A 与 _G417（内部变量名称）统一、B 与 _G418 统一、N 与 0 统一的点。。此时 A 是您的外部变量，_G417 是您的内部 A。如果此调用成功，它最终会执行 prolog 将仅报告外部变量值。在内部，_G417 从来不与其他任何东西统一。我认为问题之一是理解 Prolog 的统一模型如何工作。A is not being Unified with anything in the body of your rules. The way prolog works is via unification of terms. You cannot "return" A as in procedural languages as such. For instance, what do you want the value of A to be when the recursion comes to an end? I have no idea what your code is doing so let me use an example of my own.
Here is a sum procedure that will sum the values in a list and "return N", the sum of the values in the list. To call this procedure you can do this:
And Prolog will respond:
Notice the accumulate procedure is using A as an accumulator as the recursion goes on. That is, A keeps the running sum, while N is the final answer it returns. During the recursion N is not unified with any real value.
In the final step of the recursion, that is, when the list is empty, the value of A is unified with N, in effect returning N.
Let us do a Trace.
Now, notice the point in the trace where I marked
//A unifies with _G417 (internal variable name), B with _G418 and N with 0.. At that point A is your external variable and _G417 is your internal A. If this call succeeds which it ultimately does prolog will only report the external variable values. Internally _G417 is never unified with anything else. I think the problem is one of understanding how the unification model of Prolog works.我这里没有序言编译器，但您是否尝试过以下内容：
I don't have my prolog compiler here but have you tried something along the lines of: