Question

EDIT
Ok, @dhg discovered that dot-method syntax required if the code block to fold() is not bound to a val (why with reduce() in the same code block one can use space-method syntax, I don’t know). At any rate, the end result is the nicely concise:

result.map { row =>
  addLink( row.href, row.label )
}.fold(NodeSeq.Empty)(_++_)

Which negates to some degree the original question; i.e. in many cases one can higher-order away either/or scenarios and avoid “fat”, repetitive if/else statements.

ORIGINAL
Trying to reduce if/else handling when working with possibly empty collections like List[T]

For example, let’s say I need to grab the latest news articles to build up a NodeSeq of html news <li><a>links</a></li>:

val result = dao.getHeadlines // List[of model objects]
if(result.isEmpty) NodeSeq.Empty
else 
  result map { row =>
    addLink( row.href, row.label ) // NodeSeq
  } reduce(_ ++ _)

This is OK, pretty terse, but I find myself wanting to go ternary style to address these only-will-ever-be either/or cases:

result.isEmpty ? NodeSeq.Empty :
  result map { row =>
    addLink( row.href, row.label )
  } reduce(_ ++ _)

I’ve seen some old postings on pimping ternary onto boolean, but curious to know what the alternatives are, if any, to streamline if/else?

match {...} is, IMO, a bit bloated for this scenario, and for {...} yield doesn’t seem to help much either.

Answer 1

You don’t need to check for emptiness at all. Just use fold instead of reduce since fold allows you to specify a default “empty” value:

scala> List(1,2,3,4).map(_ + 1).fold(0)(_+_)
res0: Int = 14

scala> List[Int]().map(_ + 1).fold(0)(_+_)
res1: Int = 0

Here’s an example with a List of Seqs:

scala> List(1,2).map(Seq(_)).fold(Seq.empty)(_++_)
res14: Seq[Int] = List(1, 2)

scala> List[Int]().map(Seq(_)).fold(Seq.empty)(_++_)
res15: Seq[Int] = List()

EDIT: Looks like the problem in your sample has to do with the dropping of dot (.) characters between methods. If you keep them in, it all works:

scala> List(1,2,3).map(i => node).fold(NodeSeq.Empty)(_ ++ _)
res57: scala.xml.NodeSeq = NodeSeq(<li><a href="/foo">Link</a></li>, <li><a href="/foo">Link</a></li>, <li><a href="/foo">Link</a></li>)