scala 选项 getOrElse 类型不匹配错误
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Option getOrElse type mismatch error
提问by sndyuk
Why does this code raise a type mismatch error in Scala 2.9.2? I expected that getOrElsereturns type Stringbut actually it returns java.io.Serializable:
为什么这段代码会在 Scala 2.9.2 中引发类型不匹配错误？我期望getOrElse返回类型String但实际上它返回java.io.Serializable：
scala> implicit def StringToOption(s:String) = Option(s)
StringToOption: (s: String)Option[String]
scala> "a".getOrElse("")
res0: String = a
scala> var opt:Option[String] = "a".getOrElse("")
<console>:8: error: type mismatch;
found : java.io.Serializable
required: Option[String]
var opt:Option[String] = "a".getOrElse("")
^
This is OK:
还行吧：
scala> implicit def StringToOption(s:String): Option[String] = Option(s)
StringToOption: (s: String)Option[String]
scala> var b:Option[String] = "a".getOrElse("") toString
b: Option[String] = Some(a)
回答by Rex Kerr
It's an unwanted case of incomplete tree traversal. The signature of getOrElseallows type widening, so when it realizes that Stringis not Option[String]it first tries to fill in a different type ascription on getOrElse, i.e. Serializable. But now it has "a".getOrElse[Serializable]("")and it's stuck--it doesn't realize, I guess, that the problem was making the type too general before checking for implicits.
这是不完整的树遍历的一种不需要的情况。的签名getOrElse允许类型扩展，因此当它意识到String不是Option[String]它时，它首先尝试在 上填充不同的类型归属getOrElse，即Serializable。但是现在它已经"a".getOrElse[Serializable]("")并且卡住了 - 我猜它没有意识到问题是在检查隐式之前使类型过于笼统。
Once you realize the problem, there's a fix:
一旦你意识到问题，就有一个解决办法：
"a".getOrElse[String]("")
Now the typer doesn't wander down the let's-widen path, and finds the implicit.
现在打字机不会沿着 let's-widen 路径徘徊，而是找到隐含的。
