Scala隐式解析机制是否与声明顺序有关? [英] Scala implicits resolution mechanism is declaration order dependent?

问题描述

在每天的Scala编码过程中，我遇到一个问题，即Scala隐式解析取决于声明顺序.一个简单的例子:

During daily Scala coding I faced an issue that Scala implicits resolution depends on declaration order. A simple example:

object example extends App {
  trait FooTypeClass[T] {
    def foo: T
  }

  def bar[T](implicit tc: FooTypeClass[T]) = println(tc.foo)

  class A {
    // bar[A] doesn't compile
  }
  object A {
    implicit object aFoo extends FooTypeClass[A] {
      def foo: A = new A { override def toString = "a" }
    }
  }

  bar[A]
}

它确实可以编译，但是如果我取消注释注释行，它将找不到作用域中所需的隐式对象.因此，要使其编译，应在class A之前放置object A声明.这意味着我的同伴应该去上他们的课，但是我宁愿离开他们，就像在示例中一样.

It does compile, but if I uncomment the commented line, it won't find the required implicit in scope. So, to make it compile I should place object A declaration before class A. That means my companions should go before their classes but I'd rather leave them as they are in the example.

为什么这个顺序很重要?有解决方法吗?

Why does this order matter? Is there a workaround for this?

推荐答案

一种可能的解决方法，可以保持您希望的顺序:

A possible workaround keeping the order you wished:

object A {
    implicit val aFoo: FooTypeClass[A] = new FooTypeClass[A] {
      def foo: A = new A {
        override def toString = "a"
      }
    }
  }

我一直在寻找为什么object(而不是val)不合适的解释.

I keep on seeking for the explanation why object (instead of val) doesn't fit.

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