选项getOrElse类型不匹配错误

问题描述

为什么此代码会在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:

Why does this code raise a type mismatch error in Scala 2.9.2? I expected that getOrElse returns type String but actually it returns 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("")
                                             ^

这可以:

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)

推荐答案

这是不完整的树遍历的不良情况. getOrElse的签名允许类型扩展，因此，当意识到String不是Option[String]时，它首先尝试在getOrElse上填充不同的类型名称，即Serializable.但是现在它有了"a".getOrElse[Serializable]("")并被卡住了-我想我没有意识到问题在于在检查隐式类型之前，该类型太笼统了.

It's an unwanted case of incomplete tree traversal. The signature of getOrElse allows type widening, so when it realizes that String is 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.

一旦意识到问题，就可以解决:

Once you realize the problem, there's a fix:

"a".getOrElse[String]("")

现在，打字员无需沿着让我们扩大"的路径徘徊，而可以找到隐式内容.

Now the typer doesn't wander down the let's-widen path, and finds the implicit.

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