当方法以看似无关的方式重载时,为什么scala无法编译？

Question

class A {}
class B extends A {}

object Sample {
  def foo(a: Set[A]) {
    println("Hi Set[A]")
  }
  // def foo(a: String) {
  //   println("Hi A")
  // }
}

Sample.foo(Set(new B()))

上面的代码很愉快scala.但是,当我取消注释时foo(a: String),代码无法编译:

test.scala:13: error: overloaded method value foo with alternatives:
  (a: String)Unit <and>
  (a: Set[this.A])Unit
 cannot be applied to (scala.collection.immutable.Set[this.B])
Sample.foo(Set(new B()))
       ^
one error found

foo(a: String)似乎它应该与试图foo用一个调用无关Set[B].到底是怎么回事？

编辑:

让我感到困惑的不仅仅是为什么未注释的版本不能编译,而是为什么它在编译时会foo(a: String)被注释掉.我通过添加方法改变了什么foo(a: String)？

Set不变不能解释为何在foo(a: String)注释时成功编译.

Answer 1

在工作情况下，类型参数 toSet.apply[T]被推断为，A因为Set[A]是函数参数的预期类型。

重载解析类型检查参数没有预期的类型，因此编译器不能再用来Set[A]指导推断您想要的集合。

这是规范中的一个重要内容，尽管现在它被有关 SAM 的更多文字所掩盖。

否则，令 Si... 为通过键入每个参数获得的类型列表，如下所示。[关于函数的一些事情。] 所有其他参数都使用未定义的预期类型进行键入。

如果它知道Set[A]需要 a ，则您的集合将以这种方式键入，而不是作为Set[B]。

您可以使用 观察输入决策-Ytyper-debug，它发出的输出有时并不难以理解。

给定

class A ; class B extends A

object X { def f(as: Set[A]) = ??? ; def f(s: String) = ??? }

object Main extends App {
  X.f(Set(new B))
}

在这里，值参数的类型为Set[B]，然后它尝试找到到重载的参数类型的隐式转换，但失败了。

它还使用采用 的方法查找X对象到类型的转换。fSet[B]

|    |-- X.f(Set(new B())) BYVALmode-EXPRmode (site: value <local Main> in Main)
|    |    |-- X.f BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |-- X EXPRmode-POLYmode-QUALmode (silent: value <local Main> in Main)
|    |    |    |    \-> X.type
|    |    |    \-> (s: String)Nothing <and> (as: Set[A])Nothing
|    |    |-- Set(new B()) BYVALmode-EXPRmode (silent: value <local Main> in Main)
|    |    |    |-- Set BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |-- scala.Predef.Set.apply BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |    [adapt] [A](elems: A*)CC[A] adapted to [A](elems: A*)CC[A]
|    |    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    |    [adapt] => scala.collection.immutable.Set.type adapted to [A](elems: A*)CC[A]
|    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    |-- new B() BYVALmode-EXPRmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |-- new B BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |    |-- new B EXPRmode-POLYmode-QUALmode (silent: value <local Main> in Main)
|    |    |    |    |    |    |-- B FUNmode-TYPEmode (silent: value <local Main> in Main)
|    |    |    |    |    |    |    \-> B
|    |    |    |    |    |    \-> B
|    |    |    |    |    \-> ()B
|    |    |    |    \-> B
|    |    |    solving for (A: ?A)
|    |    |    \-> scala.collection.immutable.Set[B]
|    |    [search #1] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=scala.collection.immutable.Set[B] => String (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    [search #2] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=(=> scala.collection.immutable.Set[B]) => String (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    [search #3] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=scala.collection.immutable.Set[B] => Set[A] (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    [search #4] start `(s: String)Nothing <and> (as: Set[A])Nothing`, searching for adaptation to pt=(=> scala.collection.immutable.Set[B]) => Set[A] (silent: value <local Main> in Main) implicits disabled
|    |    15 implicits in companion scope
|    |    second try: <error> and Set(new B())
|    |    |-- Set(new B()) EXPRmode (silent: value <local Main> in Main)
|    |    |    |-- Set BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |-- scala.Predef.Set.apply BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value <local Main> in Main)
|    |    |    |    |    [adapt] [A](elems: A*)CC[A] adapted to [A](elems: A*)CC[A]
|    |    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    |    [adapt] => scala.collection.immutable.Set.type adapted to [A](elems: A*)CC[A]
|    |    |    |    \-> (elems: A*)scala.collection.immutable.Set[A]
|    |    |    solving for (A: ?A)
|    |    |    \-> scala.collection.immutable.Set[B]
|    |    [search #5] start `X.type`, searching for adaptation to pt=X.type => ?{def f(x$1: ? >: scala.collection.immutable.Set[B]): ?} (silent: value <local Main> in Main) implicits disabled
|    |    [search #6] start `X.type`, searching for adaptation to pt=(=> X.type) => ?{def f(x$1: ? >: scala.collection.immutable.Set[B]): ?} (silent: value <local Main> in Main) implicits disabled
badset.scala:7: error: overloaded method value f with alternatives:
  (s: String)Nothing <and>
  (as: Set[A])Nothing
 cannot be applied to (scala.collection.immutable.Set[B])