Java中更高级的泛型 [英] Higher-kinded generics in Java

问题描述

假设我有以下类：

  public class FixExpr {
 Expr< FixExpr>在; 
} 
  

现在我想介绍一个通用的参数，抽象使用Expr：

  public class Fix< F> {
 F<修复< F>>在; 
} 
  

但Eclipse不喜欢这样：

类型F不是通用的;它不能用参数参数化< Fix< F>>

这是可能的，还是我忽略了导致这种情况的原因有些背景信息：在Haskell中，这是编写泛型函数的常用方法;在Haskell中，我正试图将其移植到Java。上例中的类型参数F具有* - > *类型，而不是通常的类型*。在Haskell中，它看起来像这样：

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

解决方案

我认为你所要做的只是Java不支持仿制药。

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

也不使用javac编译。

由于Java在编译时并不知道 T 是什么，它不能保证 T 完全有意义。例如，如果您创建了 Foo< BufferedImage> ，那么 bar 将具有签名

  public BufferedImage< String> bar（）
  

这是无意义的。由于没有任何机制迫使你只用 T s实例化 Foo s，所以它拒绝编译。 / p>

Suppose I have the following class:

public class FixExpr {
  Expr<FixExpr> in;
}

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

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

But Eclipse doesn't like this:

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

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

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) }

解决方案

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; }
}

also does not compile using javac.

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()

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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