Option getOrElse type mismatch error

Question

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.

Answer 1

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.

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

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

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