scala - 为什么 'None' 会中断包含 'for' 的 'Option' 理解？

Question

为什么函数 personFromParams 返回 None？

  def personFromParams(p: Map[String, String]): Option[Person] =
    for {
      name <- p.get("name")
      ageStr <- p.get("age")
      age <- toInt(ageStr)
      validStr <- p.get("valid")
      valid <- toBool(validStr)
    } yield { println("personFromParams()"); new Person(name, age, valid) }

   def tryo[T](f: => T): Option[T] = try {Some(f)} catch {case _ => None}

   def toInt(s: String): Option[Int] = tryo(s.toInt);
   def toBool(s: String) = tryo(JBool.parseBoolean(s))

如果将非数字值赋给toInt，它会为age返回None，但我不明白为什么在函数tryo中捕获并处理异常时，toInt会中断for。

Answer 1

因为for理解可以等价地写成这样

p.get("name").flatMap{
  name => p.get("age").flatMap {
    ageStr => toInt(ageStr).flatMap {
      age => p.get("valid").flatMap {
        validStr => p.get("valid").flatMap {
          validStr => toBool(validStr).map{
            valid => { println("personFromParams()"); new Person(name, age, valid) }
          } 
        }
      }
    }
  }
}

来自 Option.flatMap 的 Scala 文档:

Returns the result of applying f to this Option's value if this Option is nonempty. Returns None if this Option is empty. Slightly different from map in that f is expected to return an Option (which could be None).