扩展具有需要隐式成员的特征的对象 [英] Extending an object with a trait which needs implicit member

问题描述

我正在尝试使用如下代码:

I'm trying to have a code like below:

object MetaData extends CacheParams{}

因此，由于 CacheParams 需要隐式val p:Parameters ，我尝试了:

So, since CacheParams needs implicit val p:Parameters, I tried:

对象元数据(隐式val p:参数)扩展了CacheParams

但是似乎我无法将参数传递给对象.

But it seems that I can't pass arguments to an object.

(因为它会给出错误:特征或对象可能没有参数)

( because it gives error:traits or objects may not have parameters)

如果我不传递任何参数，则会出现以下编译错误:

And if I don't pass any arguments it will give compile error that:

[error]: object creation impossible, since value p in trait CacheParams of type Parameters is not defined

我不知道该如何运作.有一些类似的问题，但是他们的答案都不能解决我的问题.任何帮助将非常感激.非常感谢.

I have no idea how to make this works. There were a few similar questions, but none of their answers solved my problem. Any help would be really appreciated. Thanks a lot.

推荐答案

如果我猜对了 CacheParams 的定义

trait Parameters

trait CacheParams {
  implicit val p: Parameters
}

然后您应该:(1)替换对象

then you should: (1) replace the object

object MetaData /*(implicit val p: Parameters)*/ extends CacheParams

//object creation impossible. Missing implementation for:
//  implicit val p: Parameters // inherited from trait CacheParams

上课

class MetaData(implicit val p: Parameters) extends CacheParams

或(2)在对象内部实现隐式

or (2) implement the implicit inside the object

object MetaData extends CacheParams {
  override implicit val p: Parameters = new Parameters {}
}

或

implicit val params: Parameters = new Parameters {} // suppose an implicit is in current scope

object MetaData extends CacheParams {
  override implicit val p: Parameters = {
    val p = ??? // hiding above p to avoid ambiguous implicits, null or NPE, see (*) below
    implicitly[Parameters]
  }
}

(*)隐式分辨率上的NullPointerException

在(1)中，您正在当前作用域中定义/解析隐式.在(2)中，将隐式解析推迟到实例化类之前(在类构造函数调用站点的范围内解析隐式).

In (1) you're defining/resolving implicit now in current scope. In (2) you postpone resolving implicit till instantiating the class (resolving implicit in the scope of class constructor call site).

另请参见通过乘法对象传递隐式参数

是的，这可以解决我的问题.但是我想保持对象的主体清洁，并使其与Parameters中的配置分离，例如接受 p:Parameters 的类的主体.

如果在(1)中，您要使对象与" Parameters 中的配置"解耦，您可以尝试引入特征( ParamsMaterializer ):

If in (1) you want to decouple the object from "the configuration in Parameters" you can try to introduce a trait (ParamsMaterializer):

object MetaData extends CacheParams with ParamsMaterializer

trait ParamsMaterializer {
  implicit val p: Parameters = new Parameters {}
}

或

implicit val params: Parameters = new Parameters {} // suppose an implicit is in current scope

object MetaData extends CacheParams with ParamsMaterializer

trait ParamsMaterializer {
  implicit val p: Parameters = {
    val p = ???
    implicitly[Parameters]
  }
}

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