CODEWITHSUNDEEP
scalapattern-matching
loyalflow
loyalflow

Reputation: 14899

When matching, do you create a variable on the fly in the case statement?

I'm looking at this scala code and confused where the variable partitionedData is coming from in the case Some(partitionedData) statement:

private def dispatchSerializedData(messages: Seq[KeyedMessage[K,Message]]): Seq[KeyedMessage[K, Message]] = {
    val partitionedDataOpt = partitionAndCollate(messages)
    partitionedDataOpt match {
      case Some(partitionedData) =>
        val failedProduceRequests = new ArrayBuffer[KeyedMessage[K,Message]]
        try {
          for ((brokerid, messagesPerBrokerMap) <- partitionedData) {
            if (logger.isTraceEnabled)
              messagesPerBrokerMap.foreach(partitionAndEvent =>
                trace("Handling event for Topic: %s, Broker: %d, Partitions: %s".format(partitionAndEvent._1, brokerid, partitionAndEvent._2)))
            val messageSetPerBroker = groupMessagesToSet(messagesPerBrokerMap)

            val failedTopicPartitions = send(brokerid, messageSetPerBroker)
            failedTopicPartitions.foreach(topicPartition => {
              messagesPerBrokerMap.get(topicPartition) match {
                case Some(data) => failedProduceRequests.appendAll(data)
                case None => // nothing
              }
            })
          }
        } catch {
          case t: Throwable => error("Failed to send messages", t)
        }
        failedProduceRequests
      case None => // all produce requests failed
        messages
    }
  }

During a match, do you create a variable on the fly? is it equal to partitionedDataOpt ?

Upvotes: 2

Views: 109

Answers (1)

Eran Medan
Eran Medan

Reputation: 45765

partitionedDataOpt is an Option and can be either a Some(value) or a None (both are subtypes of Option)

If partitionedDataOpt is a Some Option, it wraps the actual value inside, and the pattern matching marks partitionedData as the value contained in the option. In that case you can name it however you want, and it is local to the matched case.

During a match, do you create a variable on the fly?

You can say so, I think partitionedData can be seen as a local val scoped in the case clause

is it equal to partitionedDataOpt ?

You can say it's equal to partitionedDataOpt.get if partitionedDataOpt has a value (e.g. a Some)

It is not unique to Options by the way Some and None are simply case classes extending Option (source)

/** Class `Some[A]` represents existing values of type
 *  `A`.
 *
 *  @author  Martin Odersky
 *  @version 1.0, 16/07/2003
 */
final case class Some[+A](x: A) extends Option[A] {
  def isEmpty = false
  def get = x
}


/** This case object represents non-existent values.
 *
 *  @author  Martin Odersky
 *  @version 1.0, 16/07/2003
 */
case object None extends Option[Nothing] {
  def isEmpty = true
  def get = throw new NoSuchElementException("None.get")
}

Upvotes: 2

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