动态混合特征 [英] Mixing in a trait dynamically

问题描述

具有特征

trait Persisted {
  def id: Long
}

我如何实现一个接受任何case类的实例并返回其特征混合在一起的副本的方法?

how do I implement a method that accepts an instance of any case class and returns its copy with the trait mixed in?

方法的签名如下:

def toPersisted[T](instance: T, id: Long): T with Persisted

推荐答案

这可以通过宏(自2.10.0-M3以来正式成为Scala的一部分)来完成. 这是您要查找的内容的主要示例.

This can be done with macros (that are officially a part of Scala since 2.10.0-M3). Here's a gist example of what you are looking for.

1)我的宏生成一个本地类，该类继承自提供的case类并被持久化，就像new T with Persisted一样.然后，它会缓存其参数(以防止进行多次评估)并创建所创建类的实例.

1) My macro generates a local class that inherits from the provided case class and Persisted, much like new T with Persisted would do. Then it caches its argument (to prevent multiple evaluations) and creates an instance of the created class.

2)我怎么知道要生成什么树?我有一个简单的应用程序parse.exe，该应用程序打印由解析输入代码产生的AST.因此，我只调用了parse class Person$Persisted1(first: String, last: String) extends Person(first, last) with Persisted，记下了输出并在宏中复制了它. parse.exe是scalac -Xprint:parser -Yshow-trees -Ystop-after:parser的包装.有多种探索AST的方法，请参见在Scala 2.10中进行元编程" .

2) How did I know what trees to generate? I have a simple app, parse.exe that prints the AST that results from parsing input code. So I just invoked parse class Person$Persisted1(first: String, last: String) extends Person(first, last) with Persisted, noted the output and reproduced it in my macro. parse.exe is a wrapper for scalac -Xprint:parser -Yshow-trees -Ystop-after:parser. There are different ways to explore ASTs, read more in "Metaprogramming in Scala 2.10".

3)如果您提供-Ymacro-debug-lite作为scalac的参数，则可以检查宏扩展.在这种情况下，所有扩展都将被打印出来，您将能够更快地检测到代码生成错误.

3) Macro expansions can be sanity-checked if you provide -Ymacro-debug-lite as an argument to scalac. In that case all expansions will be printed out, and you'll be able to detect codegen errors faster.

编辑.更新了2.10.0-M7的示例

edit. Updated the example for 2.10.0-M7

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