Scala for comprehension of sequence inside a Try - scala

Question

I am writing a Scala program in which there is an operation that creates a sequence. The operation might fail, so I enclose it inside a Try. I want to do sequence creation and enumeration inside a for comprehension, so that a successfully-created sequence yields a sequence of tuples where the first element is the sequence and the second is an element of it.
To simplify the problem, make my sequence a Range of integers and define a createRange function that fails if it is asked to create a range of an odd length. Here is a simple for comprehension that does what I want.
import scala.util.Try
def createRange(n: Int): Try[Range] = {
Try {
if (n % 2 == 1) throw new Exception
else Range(0, n)
}
}
def rangeElements(n: Int) {
for {
r <- createRange(n)
x <- r
} println(s"$r\t$x")
}
def main(args: Array[String]) {
println("Range length 3")
rangeElements(3)
println("Range length 4")
rangeElements(4)
}
If you run this it correctly prints.
Range length 3
Range length 4
Range(0, 1, 2, 3) 0
Range(0, 1, 2, 3) 1
Range(0, 1, 2, 3) 2
Range(0, 1, 2, 3) 3
Now I would like to rewrite my rangeElements function so that instead of printing as a side-effect it returns a sequence of integers, where the sequence is empty if the range was not created. What I want to write is this.
def rangeElements(n: Int):Seq[(Range,Int)] = {
for {
r <- createRange(n)
x <- r
} yield (r, x)
}
// rangeElements(3) returns an empty sequence
// rangeElements(4) returns the sequence (Range(0,1,2,3), 0), (Range(0,1,2,3), 1) etc.
This gives me two type mismatch compiler errors. The r <- createRange(n) line required Seq[Int] but found scala.util.Try[Nothing]. The x <- r line required scala.util.Try[?] but found scala.collection.immutable.IndexedSeq[Int].
Presumably there is some type erasure with the Try that is messing me up, but I can't figure out what it is. I've tried various toOption and toSeq qualifiers on the lines in the for comprehension to no avail.
If I only needed to yield the range elements I could explicitly handle the Success and Failure conditions of createRange myself as suggested by the first two answers below. However, I need access to both the range and its individual elements.
I realize this is a strange-sounding example. The real problem I am trying to solve is a complicated recursive search, but I don't want to add in all its details because that would just confuse the issue here.
How do I write rangeElements to yield the desired sequences?

Answer 1

The problem becomes clear if you translate the for comprehension to its map/flatMap implementation (as described in the Scala Language Spec 6.19). The flatMap has the result type Try[U] but your function expects Seq[Int].
for {
r <- createRange(n)
x <- r
} yield x
createRange(n).flatMap {
case r => r.map {
case x => x
}
}
Is there any reason why you don't use the getOrElse method?
def rangeElements(n: Int):Seq[Int] =
createRange(n) getOrElse Seq.empty

Answer 2

The Try will be Success with a Range when n is even or a Failure with an Exception when n is odd. In rangeElements match and extract those values. Success will contain the valid Range and Failure will contain the Exception. Instead of returning the Exception return an empty Seq.
import scala.util.{Try, Success, Failure}
def createRange(n: Int): Try[Range] = {
Try {
if (n % 2 == 1) throw new Exception
else Range(0, n)
}
}
def rangeElements(n: Int):Seq[Tuple2[Range, Int]] = createRange(n) match {
case Success(s) => s.map(xs => (s, xs))
case Failure(f) => Seq()
}
scala> rangeElements(3)
res35: Seq[(Range, Int)] = List()
scala> rangeElements(4)
res36: Seq[(Range, Int)] = Vector((Range(0, 1, 2, 3),0), (Range(0, 1, 2, 3),1), (Range(0, 1, 2, 3),2), (Range(0, 1, 2,3),3))