Scala - can 'for-yield' clause yields nothing for some condition?

Question

In Scala language, I want to write a function that yields odd numbers within a given range. The function prints some log when iterating even numbers. The first version of the fu

Answer 1

If you will to dig into a functional approach, something like the following is a good point to start.

First some common definitions:

    // use scalaz 7
    import scalaz._, Scalaz._

    // transforms a function returning either E or B into a 
    // function returning an optional B and optionally writing a log of type E
    def logged[A, E, B, F[_]](f: A => E \/ B)(
      implicit FM: Monoid[F[E]], FP: Pointed[F]): (A => Writer[F[E], Option[B]]) = 
      (a: A) => f(a).fold(
        e => Writer(FP.point(e), None), 
        b => Writer(FM.zero, Some(b)))

    // helper for fixing the log storage format to List
    def listLogged[A, E, B](f: A => E \/ B) = logged[A, E, B, List](f)

    // shorthand for a String logger with List storage
    type W[+A] = Writer[List[String], A]

Now all you have to do is write your filtering function:

    def keepOdd(n: Int): String \/ Int = 
      if (n % 2 == 1) \/.right(n) else \/.left(n + " was even")

You can try it instantly:

    scala> List(5, 6) map(keepOdd)
    res0: List[scalaz.\/[String,Int]] = List(\/-(5), -\/(6 was even))

Then you can use the traverse function to apply your function to a list of inputs, and collect both the logs written and the results:

    scala> val x = List(5, 6).traverse[W, Option[Int]](listLogged(keepOdd))
    x: W[List[Option[Int]]] = scalaz.WriterTFunctions$$anon$26@503d0400

    // unwrap the results
    scala> x.run
    res11: (List[String], List[Option[Int]]) = (List(6 was even),List(Some(5), None))

    // we may even drop the None-s from the output
    scala> val (logs, results) = x.map(_.flatten).run
    logs: List[String] = List(6 was even)
    results: List[Int] = List(5)