使用 ==/2 或 diff/2 [英] Using ==/2 or dif/2
问题描述
如果我想确保两个变量不会实例化为同一个术语，那么首选的方法是什么?
假设我需要在图中找到有向边，而一个节点自身不能有边:
节点(a，x，y).节点(b，z，x).节点(c，y，y).(这里的边是a -> c，b -> a，但不是 c -> c)
以下作品:
edge(A, B) :- 节点(A, _, X), 节点(B, X, _), A == B.这也有效 [swi-prolog]:
edge(A, B) :- diff(A, B), node(A, _, X), node(B, X, _).这显然不起作用(因为 A 和 B 都没有被实例化?):
edge(A, B) :- A == B, node(A, _, X), node(B, X, _).我想我对第一个解决方案的问题是，使用更复杂的 node 谓词，在 edge 失败之前可能会发生许多不必要的统一.另一方面，dif 位于库中，这表明它不适合在如此简单的情况下使用(尽管它具有我似乎正在寻找的确切功能).
仅出于优雅和教学的原因，dif/2 显然在这里以及绝大多数其他情况下更可取，因为您已经注意到可能会发生许多不必要的统一"，而且还因为 dif/2 是一个纯粹且很好的声明性谓词，可以在所有方向和子句主体的任何位置使用与 (==)/2 相比，不改变程序的含义.dif/2 也是 SWI-Prolog 中的自动加载谓词，这意味着您需要不显式导入任何库来使用它，并且 dif/2 像任何内置谓词一样可用.
如果您使用 dif/2，您可以更轻松地推断您的代码.例如，在你的情况下，你开始:
然后，你知道 dif/2 是一个完全纯谓词，你知道你也可以这样写:
此外，由于您知道 dif/2 总是终止，因此您知道此更改最多可以改善程序的终止属性.
与所有约束一样，dif/2 是要使用的.我强烈推荐它而不是不可交换的不纯谓词.
如果您担心性能，这里有一个小比较，只是将 dif/2 与非声明性 (==)/2 进行比较两个谓词可以互换使用的用例:
因此，使用 (==)/2 有时会带来性能优势.然而，使用这种低级谓词时也有更严重的缺点:更难理解、更容易出错，而且不是声明性的.
因此我建议简单地使用 dif/2 来表示两个术语不同.
If I want to make sure that two variables do not instantiate to the same term, what is the preferred way to do it?
Let's say I need to find directed edges in a graph, and a node cannot have an edge to itself:
node(a, x, y). node(b, z, x). node(c, y, y).
(the edges here are a -> c, b -> a, but not c -> c)
The following works:
edge(A, B) :- node(A, _, X), node(B, X, _), A == B.
This works too [swi-prolog]:
edge(A, B) :- dif(A, B), node(A, _, X), node(B, X, _).
This does not work, apparently (because neither A nor B are instantiated yet?):
edge(A, B) :- A == B, node(A, _, X), node(B, X, _).
I guess my problem with the first solution is that, with a more complex node predicate, a lot of unnecessary unifications might take place before edge fails. The dif on the other hand is in a library, which suggests that it is not meant to be used in such a simple case (although it has the exact function that I seem to be looking for).
For elegance and didactic reasons alone, dif/2 is clearly preferable here and also in the vast majority of other cases, since as you already note "a lot of unnecessary unifications might take place" otherwise, and also because dif/2 is a pure and nicely declarative predicate that can be used in all directions and at any place in the clause body without changing the meaning of the program, in contrast to (==)/2. dif/2 is also an autoloaded predicate in SWI-Prolog, meaning that you need not import any library explicitly to use it, and dif/2 is available like any built-in predicate.
If you use dif/2 you can reason much more easily about your code. For example, in your case, you start with:
edge(A, B) :- node(A, _, X), node(B, X, _), dif(A, B).
and then, as you know that dif/2 is a completely pure predicate, you know that you can also write this as:
edge(A, B) :- dif(A, B), node(A, _, X), node(B, X, _).
Further, since you know that dif/2 always terminates, you know that this change can at most improve the termination properties of your program.
Like all constraints, dif/2 is meant to be used. I highly recommend it instead of impure predicates that are not commutative.
In case you are worried about performance, here is a small comparison, just comparing dif/2 against the non-declarative (==)/2 in a use case where the two predicates can be used interchangeably:
?- N = 1_000_000, time((between(1,N,_),dif(a,b),false)). % 11,000,005 inferences, 0.352 CPU in 0.353 seconds (100% CPU, 31281029 Lips) ?- N = 1_000_000, time((between(1,N,_),a==b,false)). %@ % 3,000,001 inferences, 0.107 CPU in 0.107 seconds (99% CPU, 28167437 Lips)
So, there are sometimes performance benefits when using (==)/2. However, there are also much more severe drawbacks when using such a low-level predicate: It is harder to understand, more error-prone, and not declarative.
I therefore recommend to simply use dif/2 to express that two terms are different.
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