I have a list of string ids representing DB records. I'd like to load them from the DB asynchronously, then upload each record to a remote server asynchronously, then when all are done uploading, make a record of the ids of the records that were uploaded.

Since I'm on Scala 2.9.2, I'm using Twitter's core-util Future implementation, but it should work exactly like the 2.10 futures in terms of Monadic transformations.

The general concept is this:

def fetch(id: String): Future[Option[Record]]
def upload(record: Record): Future[String]
def notifyUploaded(ids: Seq[String]): Unit

val ids: Seq[String] = ....

I'm trying to do this via a for comprehension but the fact that fetch returns a Future of Option makes it obscure and the code doesn't compile:

for {
  id <- ids
  maybeRecord <- fetch(id)
  record <- maybeRecord
  uploadedId <- upload(record)
} yield uploadedId

Compiling this results in the following error:

scala: type mismatch;
found   : com.twitter.util.Future[String]
required: Option[?]
    uploadedId <- upload(record)
                  ^

What am I missing? why does the compiler expect uploadedId to be an Option? is there any pretty way I could work around this?

scala monads future
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