如何在Prolog中传递谓词作为另一个谓词的参数？ [英] How can I pass a predicate as parameter for another predicate in Prolog?

问题描述

我有这3个谓词：

times(X, Y):-
    Result is X * Y.
minus(X, Y):-
    Result is X - Y.
plus(X, Y):-
    Result is X + Y.

，我想通过例如 times（2,2）在加（X，Y）像这样加（times（2,2），minus（X，Y））。

and I want to pass for example times(2,2) in the plus(X, Y) like this plus(times(2,2), minus(X, Y)).

推荐答案

您的问题标题与您的问题的文字之间的关系对我来说不清楚，我认为@false可能是正确的，这里有一个更基本的误解Prolog。我不知道这是否真的满足了你的需要，但这里的替代方法就是编写自己的评估者。

The relationship between the title of your question and the text of your question is unclear to me, and I think @false is probably right that there is a more fundamental misunderstanding about Prolog here. I don't know if this really addresses your need or not, but the alternative here is writing your own evaluator.

eval(times(X,Y), Result) :-
    eval(X, XResult),
    eval(Y, YResult),
    Result is XResult * YResult.
eval(minus(X,Y), Result) :-
    eval(X, XResult),
    eval(Y, YResult),
    Result is XResult - YResult.
eval(plus(X,Y), Result) :-
    eval(X, XResult),
    eval(Y, YResult),
    Result is XResult + YResult.

递归调用 eval / 2 需要这些规则中的每一个的身体来处理例如 plus（times（2,2），minus（X，Y））的情况。那么你需要一个数字规则：

The recursive calls to eval/2 inside the bodies of each of these rules are needed to handle cases like plus(times(2,2), minus(X, Y)). Then you need a rule for numbers:

eval(Num, Num) :- number(Num).

这对于像这样的情况非常好：

This works great for cases like this:

?- eval(plus(times(2,2), minus(7,1)), Result).
Result = 10.

对于这样的情况，你不会有任何好处：

It doesn't do you any good for cases like this:

?- eval(plus(times(2,2), minus(X,Y)), Result).
ERROR: Out of local stack

当然，如果我们建立绑定X和Y在到达之前，但如果您希望它为X和Y生成可能的解决方案，那么您将不幸运，您需要使用 clpfd 。这个奇怪的错误的原因，如果你追踪，是因为 number（X）当 X 未绑定是false ，所以它实际上生成了涉及时间，减号和加号结构的新条款，并尝试它们，这不是您在评估者中所需要的。

Of course, it would work if we established bindings for X and Y before getting there, but if you want it to generate possible solutions for X and Y you're out of luck, you'll need to use clpfd. The reason for this curious error, if you trace in, is because number(X) when X is unbound is false, so it is actually generating new clauses involving the times, minus and plus structures and trying them, which isn't what you want in an evaluator.

编辑 c

eval / 2 谓词显示如何执行递归树步行。校长与制作漂亮的打印机相同。我非常懒惰，所以我只会草绘，你必须自己填写细节。

The eval/2 predicate shows you how to perform a recursive tree walk. The principal is the same with making a pretty printer. I am very lazy so I will only sketch it, you'll have to fill in the details yourself.

printterm(T) :- format_term(T, Formatted), write(Formatted), nl.

format_term(plus(X,Y), Formatted) :- 
  format_term(X, XFormatted),
  format_term(Y, YFormatted),
  format(atom(Formatted), '(~a + ~a)', [XFormatted, YFormatted]).

% other format_term clauses here for other arithmetic expressions

format_term(X, X) :- number(X).

希望这有帮助！

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