有没有办法比较 lambdas? [英] Is there a way to compare lambdas?

问题描述

假设我有一个使用 lambda 表达式(闭包)定义的对象列表.有没有办法检查它们以便进行比较?

Say I have a List of object which were defined using lambda expressions (closures). Is there a way to inspect them so they can be compared?

我最感兴趣的代码是

    List<Strategy> strategies = getStrategies();
    Strategy a = (Strategy) this::a;
    if (strategies.contains(a)) { // ...

完整代码是

import java.util.Arrays;
import java.util.List;

public class ClosureEqualsMain {
    interface Strategy {
        void invoke(/*args*/);
        default boolean equals(Object o) { // doesn't compile
            return Closures.equals(this, o);
        }
    }

    public void a() { }
    public void b() { }
    public void c() { }

    public List<Strategy> getStrategies() {
        return Arrays.asList(this::a, this::b, this::c);
    }

    private void testStrategies() {
        List<Strategy> strategies = getStrategies();
        System.out.println(strategies);
        Strategy a = (Strategy) this::a;
        // prints false
        System.out.println("strategies.contains(this::a) is " + strategies.contains(a));
    }

    public static void main(String... ignored) {
        new ClosureEqualsMain().testStrategies();
    }

    enum Closures {;
        public static <Closure> boolean equals(Closure c1, Closure c2) {
            // This doesn't compare the contents 
            // like others immutables e.g. String
            return c1.equals(c2);
        }

        public static <Closure> int hashCode(Closure c) {
            return // a hashCode which can detect duplicates for a Set<Strategy>
        }

        public static <Closure> String asString(Closure c) {
            return // something better than Object.toString();
        }
    }    

    public String toString() {
        return "my-ClosureEqualsMain";
    }
}

看来唯一的解决方案是将每个 lambda 定义为一个字段并仅使用这些字段.如果要打印出调用的方法，最好使用 Method.有没有更好的 lambda 表达式方法?

It would appear the only solution is to define each lambda as a field and only use those fields. If you want to print out the method called, you are better off using Method. Is there a better way with lambda expressions?

此外，是否可以打印 lambda 并获得人类可读的内容?如果你打印 this::a 而不是

Also, is it possible to print a lambda and get something human readable? If you print this::a instead of

ClosureEqualsMain$$Lambda$1/821270929@3f99bd52

得到类似的东西

ClosureEqualsMain.a()

甚至使用 this.toString 和方法.

my-ClosureEqualsMain.a();

推荐答案

这个问题可以相对于规范或实现来解释.显然，实现可能会发生变化，但您可能愿意在发生这种情况时重写您的代码，所以我会同时回答.

This question could be interpreted relative to the specification or the implementation. Obviously, implementations could change, but you might be willing to rewrite your code when that happens, so I'll answer at both.

这也取决于你想做什么.您是在寻求优化，还是在寻找可靠的保证，即两个实例是(或不是)相同的功能?(如果是后者，你会发现自己与计算物理学不一致，因为即使是问两个函数是否计算同一件事这样简单的问题也是不可判定的.)

It also depends on what you want to do. Are you looking to optimize, or are you looking for ironclad guarantees that two instances are (or are not) the same function? (If the latter, you're going to find yourself at odds with computational physics, in that even problems as simple as asking whether two functions compute the same thing are undecidable.)

从规范的角度来看，语言规范仅承诺对 lambda 表达式求值(而不是调用)的结果是实现目标功能接口的类的实例.它对结果的身份或混叠程度不作任何承诺.这是设计使然，为实现提供最大的灵活性以提供更好的性能(这就是 lambdas 比内部类更快的原因；我们不受内部类的必须创建唯一实例"约束.)

From a specification perspective, the language spec promises only that the result of evaluating (not invoking) a lambda expression is an instance of a class implementing the target functional interface. It makes no promises about the identity, or degree of aliasing, of the result. This is by design, to give implementations maximal flexibility to offer better performance (this is how lambdas can be faster than inner classes; we're not tied to the "must create unique instance" constraint that inner classes are.)

所以基本上，规范没有给你太多，除了显然两个引用相等 (==) 的 lambda 将计算相同的函数.

So basically, the spec doesn't give you much, except obviously that two lambdas that are reference-equal (==) are going to compute the same function.

从实现的角度来看，您可以得出更多结论.实现 lambda 表达式的合成类与程序中的捕获站点之间存在(当前，可能会改变)1:1 的关系.所以捕获x ->"的两个单独的代码位x + 1"很可能映射到不同的类.但是，如果您在同一个捕获点计算相同的 lambda，并且该 lambda 是非捕获的，您将获得相同的实例，可以将其与引用相等进行比较.

From an implementation perspective, you can conclude a little more. There is (currently, may change) a 1:1 relationship between the synthetic classes that implement lambdas, and the capture sites in the program. So two separate bits of code that capture "x -> x + 1" may well be mapped to different classes. But if you evaluate the same lambda at the same capture site, and that lambda is non-capturing, you get the same instance, which can be compared with reference equality.

如果您的 lambda 表达式是可序列化的，它们将更容易放弃它们的状态，以换取牺牲一些性能和安全性(没有免费的午餐.)

If your lambdas are serializable, they'll give up their state more easily, in exchange for sacrificing some performance and security (no free lunch.)

调整相等性定义可能可行的一个领域是方法引用，因为这将使它们能够用作侦听器并正确注销.正在考虑中.

One area where it might be practical to tweak the definition of equality is with method references because this would enable them to be used as listeners and be properly unregistered. This is under consideration.

我认为你想要得到的是:如果两个 lambdas 被转换为相同的功能接口，由相同的行为函数表示，并且具有相同的捕获参数，它们是相同的

I think what you're trying to get to is: if two lambdas are converted to the same functional interface, are represented by the same behavior function, and have identical captured args, they're the same

不幸的是，这既很难做到(对于不可序列化的 lambdas，你不能得到它的所有组件)而且还不够(因为两个单独编译的文件可以将相同的 lambda 转换为相同的功能接口类型，而你将无法分辨.)

Unfortunately, this is both hard to do (for non-serializable lambdas, you can't get at all the components of that) and not enough (because two separately compiled files could convert the same lambda to the same functional interface type, and you wouldn't be able to tell.)

EG 讨论了是否要公开足够的信息来做出这些判断，以及讨论 lambdas 是否应该实现更具选择性的 equals/hashCode 或更具描述性的 toString.结论是，我们不愿意为向调用者提供这些信息而付出任何性能成本(糟糕的权衡，因为有 0.01% 的好处而惩罚 99.99% 的用户).

The EG discussed whether to expose enough information to be able to make these judgments, as well as discussing whether lambdas should implement more selective equals/hashCode or more descriptive toString. The conclusion was that we were not willing to pay anything in performance cost to make this information available to the caller (bad tradeoff, punishing 99.99% of users for something that benefits .01%).

关于 toString 的最终结论尚未达成，但仍有待将来重新审视.然而，双方在这个问题上都有一些很好的论据；这不是灌篮.

A definitive conclusion on toString was not reached but left open to be revisited in the future. However, there were some good arguments made on both sides on this issue; this is not a slam-dunk.

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