当方法以看似无关的方式重载时,为什么scala无法编译？

发布时间：2020-12-16 19:16:36 所属栏目：安全来源：网络整理

导读：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())) 上面的代码与scala愉快地运行.但是,当我取消注释foo(a：String)时,代码无法编译：

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()))

上面的代码与scala愉快地运行.但是,当我取消注释foo(a：String)时,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)似乎与尝试使用Set [B]调用foo无关.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,而是为什么它会在foo(a：String)被注释掉时进行编译.通过添加方法foo(a：String)我改变了什么？

设置为不变量并不能解释当foo(a：String)被注释掉时成功编译的原因.

解决方法

在工作的情况下,Set.apply [T]的类型参数被推断为A,因为Set [A]是函数参数的预期类型.

重载决策类型检查没有预期类型的??参数,因此编译器不能再使用Set [A]来指导您想要的集合的推断.

这是the spec的一个重要收获,尽管现在有更多关于SAM的言论埋没了.

Otherwise,let Si… be the list of types obtained by typing each
argument as follows. [Something about functions.] All other arguments
are typed with an undefined expected type.

如果它知道预期有一个Set [A],那么你的集合就是这样输入的,而不是Set [B].

您可以使用-Ytyper-debug观察输入决策,它会发出有时不易察觉的输出.

特定

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))
}

这里,value参数被输入为Set [B],然后它尝试并且无法找到对重载的param类型的隐式转换.

它还寻找将X对象转换为带有F方法的类型,该方法采用Set [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])

（编辑：李大同）

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