Question

Why does function personFromParams return 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))

If give non-number value into toInt, it returns None for age, but I don’t understand
why does toInt interrupt for when the exception is caught and processed in function tryo.

Answer 1

Because the for comprehension can be equivalently written like this

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) }
          } 
        }
      }
    }
  }
}

From the scala documentation for Option.flatMap:

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).