scala-arm：Iterator [Int]类型的表达式不符合预期的ManagedResource [B_]类型

Question

此代码使用"com.jsuereth" %% "scala-arm" % "2.0"：

import resource._
val lineLengths = for {
  source <- managed(scala.io.Source.fromFile("file.txt"))
  line <- source.getLines()
} yield line.length

编译失败并显示错误：

Expression of type Iterator[Int] doesn't conform to expected type ManagedResource[B_]

有什么主意吗？