Converting a Scala implicit conversion method to a 2.10 implicit class

Question

I ma trying to convert the following Scala 2.9 implicit conversion method to a 2.10 implicit class:

import java.sql.ResultSet

/**
 * Implicitly convert a ResultSet to a Stream[ResultSet]. The Stream can then be
 * traversed using the usual map, filter, etc.
 *
 * @param row the Result to convert
 * @return a Stream wrapped around the ResultSet
 */
implicit def stream(row: ResultSet): Stream[ResultSet] = {
  if (row.next) Stream.cons(row, stream(row))
  else {
    row.close()
    Stream.empty
  }
}

My first attempt does not compile:

implicit class ResultSetStream(row: ResultSet) {
  def stream: Stream[ResultSet] = {
    if (row.next) Stream.cons(row, stream(row))
    else {
      row.close()
      Stream.empty
    }
  }
}

I get a syntax error on stream(row) because stream does not take a parameter.

What is the correct way to do this?

twillouer twillouer · Accepted Answer · 2013-02-05T17:55:40

Try this :

scala> import java.sql.ResultSet
import java.sql.ResultSet

scala> implicit class ResultSetStream(row: ResultSet) {
     |     def stream: Stream[ResultSet] = {
     |       if (row.next) Stream.cons(row, row.stream)
     |       else {
     |         row.close()
     |         Stream.empty
     |       }
     |     }
     |   }
defined class ResultSetStream

You defined stream as function, so stream(row) can not work.

You can inherit from AnyVal to create a Value Class and optimize your code:

implicit class ResultSetStream(val row: ResultSet) extends AnyVal {
    def stream: Stream[ResultSet] = {
      if (row.next) Stream.cons(row, row.stream)
      else {
        row.close()
        Stream.empty
      }
    }
  }

Converting a Scala implicit conversion method to a 2.10 implicit class

1 Answers