scala – 使用Map#getOrElse键入怪异

发布时间：2020-12-16 09:08:53 所属栏目：安全来源：网络整理

导读：考虑 scala val m = Map('a - 3,'b - 4)m: scala.collection.immutable.Map[Symbol,Int] = Map('a - 3,'b - 4)scala val d: Double = m.getOrElse('c,0)console:8: error: type mismatch; found : AnyVal required: Double val d: Double = m.getOrElse('c,0

考虑

scala> val m = Map('a -> 3,'b -> 4)
m: scala.collection.immutable.Map[Symbol,Int] = Map('a -> 3,'b -> 4)

scala> val d: Double = m.getOrElse('c,0)
<console>:8: error: type mismatch;
 found   : AnyVal
 required: Double
       val d: Double = m.getOrElse('c,0)
                                  ^

scala> m.getOrElse('c,0)
res0: Int = 0

scala> m.getOrElse('a,0)
res1: Int = 3

为什么Scala认为getOrElse调用返回AnyVal,即使它显然返回一个Int？

此外,即使这失败也会出现同样的错误：

scala> val x: Double = m.getOrElse('a,0): Double
<console>:8: error: type mismatch;
 found   : AnyVal
 required: Double
       val x: Double = m.getOrElse('a,0): Double

但这有效：

scala> val x: Double = m.getOrElse('a,0): Int
x: Double = 3.0

这发生在2.11.x;我没有在2.10.x上试过它.

解决方法

我正试图在阅读-Ytyper-debug输出方面做得更好.

对于期望的Double类型,0的类型取为Double,0.0.这是因为人们讨厌的转换次数不断扩大.

这是在解决B1之前. B1被视为AnyVal,因为它是A / Int和Double的lub.

|    |-- Double TYPEmode (site: value d  in $iw) 
|    |    -> Double
|    |-- m.getOrElse(scala.Symbol("c"),0) : pt=Double BYVALmode-EXPRmode (site: value d  in $iw) 
|    |    |-- m.getOrElse BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value d  in $iw) 
|    |    |    |-- m EXPRmode-POLYmode-QUALmode (silent: value d  in $iw) 
|    |    |    |    -> m.type (with underlying type scala.collection.immutable.Map[Symbol,Int])
|    |    |    [adapt] [B1 >: B](key: A,default: => B1)B1 adapted to [B1 >: B](key: A,default: => B1)B1
|    |    |    -> (key: Symbol,default: => B1)B1
|    |    |-- scala.Symbol("c") : pt=Symbol BYVALmode-EXPRmode-POLYmode (site: value d  in $iw) 
|    |    |    |-- scala.Symbol BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value d  in $iw) 
|    |    |    |    |-- scala.Symbol.apply BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value d  in $iw) 
|    |    |    |    |    -> (name: String)Symbol
|    |    |    |    [adapt] Symbol.type adapted to (name: String)Symbol
|    |    |    |    -> (name: String)Symbol
|    |    |    |-- "c" : pt=String BYVALmode-EXPRmode (silent: value d  in $iw) 
|    |    |    |    -> String("c")
|    |    |    -> Symbol
|    |    |-- 0 : pt=Double EXPRmode-POLYmode (site: value d  in $iw) 
|    |    |    -> Double(0.0)
|    |    solving for (B1: ?B1) 
<console>:8: error: type mismatch;
 found   : AnyVal
 required: Double
       val d: Double = m.getOrElse('c,0)
                                  ^
|    |    -> <error>

另外,-xprint：typer显示

m.getOrElse[AnyVal](scala.Symbol.apply("c"),0.0);

spec explains(案例3方法)args是使用预期的param类型进行类型检查的,这就是为什么你得到转换为Double的原因,而不是让方法返回在赋值中扩展为Double的Int.

（编辑：李大同）

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