为什么Scala不会将此类型Lambda与基础类型统一? [英] Why scala does not unify this type lambda with underlying type?

问题描述

trait A {
  type T
  def test(t: T): Unit
}

case class B[S <: A](a: S, t : S#T) {
  def test() = a.test(t) // Error: type mismatch;
    // found   : B.this.t.type (with underlying type S#T)
    // required: B.this.a.T
}

我期望以上内容编译正确吗?我的代码可以固定吗?

Am I wrong to expect the above to compile? Can my code be fixed?

推荐答案

编译器没有足够的证据证明在特定实例中S#T可以用作test的参数.

Compiler has not sufficient evidence that S#T can be used as argument for test in concrete instance.

考虑这个弱化scala编译器的过分示例

Consider this hypotecical example for weakened scala compiler

trait A2 extends A{
  type T <: AnyRef
}

class A3 extends A2{
  override type T = Integer

  def test(t: Integer): Unit = println(t * 2)
}

因此B[A2]应该接受A3的实例以及<: AnyRef的任何内容，而A3恰好需要Integer来实现自己的test

So B[A2] should accept instance of A3 along with anything that is <: AnyRef while A3 needs exactly Integer for its own test implementation

您可以在B的定义中捕获具体类型，以确保将使用哪种类型

You can catch concrete type in the definition of B, to make sure what type will be used

case class B[S <: A, ST](a: S {type T = ST}, t: ST) {
  def test() = a.test(t) 
}

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