Java 中的高级泛型 [英] Higher-kinded generics in Java

问题描述

假设我有以下课程:

public class FixExpr {
  Expr<FixExpr> in;
}

现在我想介绍一个泛型参数，对 Expr 的使用进行抽象:

Now I want to introduce a generic argument, abstracting over the use of Expr:

public class Fix<F> {
  F<Fix<F>> in;
}

但是 Eclipse 不喜欢这样:

But Eclipse doesn't like this:

类型 F 不是泛型；它不能用参数进行参数化 <Fix<F>>

The type F is not generic; it cannot be parametrized with arguments <Fix<F>>

这是可能的还是我忽略了导致此特定实例中断的某些事情?

Is this possible at all or have I overlooked something that causes this specific instance to break?

一些背景信息:在 Haskell 中，这是编写泛型函数的常用方法；我正在尝试将其移植到 Java.上面例子中的类型参数 F 有 kind * -> * 而不是通常的 kind *.在 Haskell 中，它看起来像这样:

Some background information: in Haskell this is a common way to write generic functions; I'm trying to port this to Java. The type argument F in the example above has kind * -> * instead of the usual kind *. In Haskell it looks like this:

newtype Fix f = In { out :: f (Fix f) }

推荐答案

我认为 Java 泛型根本不支持您尝试做的事情.更简单的情况

I think what you're trying to do is simply not supported by Java generics. The simpler case of

public class Foo<T> {
    public T<String> bar() { return null; }
}

也不使用 javac 编译.

also does not compile using javac.

由于 Java 在编译时不知道 T 是什么，所以不能保证 T 完全有意义.例如，如果您创建了一个 Foo，bar 将具有签名

Since Java does not know at compile-time what T is, it can't guarantee that T<String> is at all meaningful. For example if you created a Foo<BufferedImage>, bar would have the signature

public BufferedImage<String> bar()

这是无稽之谈.由于没有机制强制您仅使用通用 T 实例化 Foos，因此它拒绝编译.

which is nonsensical. Since there is no mechanism to force you to only instantiate Foos with generic Ts, it refuses to compile.

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