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POScala for comprehension of sequence inside a Try
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copied!<p>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 <code>Try</code>. 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.</p> <p>To simplify the problem, make my sequence a <code>Range</code> of integers and define a <code>createRange</code> 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.</p> <pre><code>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) } </code></pre> <p>If you run this it correctly prints.</p> <pre><code>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 </code></pre> <p>Now I would like to rewrite my <code>rangeElements</code> 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.</p> <pre><code>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. </code></pre> <p>This gives me two type mismatch compiler errors. The <code>r <- createRange(n)</code> line required <code>Seq[Int]</code> but found <code>scala.util.Try[Nothing]</code>. The <code>x <- r</code> line required <code>scala.util.Try[?]</code> but found <code>scala.collection.immutable.IndexedSeq[Int]</code>.</p> <p>Presumably there is some type erasure with the <code>Try</code> that is messing me up, but I can't figure out what it is. I've tried various <code>toOption</code> and <code>toSeq</code> qualifiers on the lines in the for comprehension to no avail.</p> <p>If I only needed to yield the range elements I could explicitly handle the <code>Success</code> and <code>Failure</code> conditions of <code>createRange</code> myself as suggested by the first two answers below. However, I need access to both the range and its individual elements.</p> <p>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.</p> <p>How do I write <code>rangeElements</code> to yield the desired sequences?</p>

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