相关疑难解决方法(0)

我每晚在2.9.1和2.10中得到不同的行为 - 改变了什么？

Welcome to Scala version 2.9.1.final (OpenJDK Client VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> Some(3) map (x => List(x, -x)) flatten
res0: List[Int] = List(3, -3)

与:

Welcome to Scala version 2.10.0.r26084-b20111129020255 (OpenJDK Client VM, Java 1.6.0_22).
Type in expressions to have them evaluated.
Type :help for more information.

scala> Some(3) map (x => List(x, -x)) flatten
<console>:8: error: Cannot prove that List[Int] <:< Option[B].
              Some(3) map (x …