Getting the element from a 1-element Scala collection

Question

Learning Scala and I keep wanting an equivalent to LINQ's Single() method. Example,

val collection: Seq[SomeType]
val (desiredItem, theOthers) = collection.partition(MyFunc)

desiredItem.single.doSomething
         // ^^^^^^

I could use desiredItem.head but what if MyFunc actually matched several? I want the assurance that there's only one.

Edit #2 The duplicate question says 'no there isn't but here's how to build it'. So I am thinking if this was a common need it would be in the base API. Do properly written Scala programs need this?

Answer 1

I'd use something more verbose instead of single:

 (desiredItem match {
   case Seq(single) => single
   case _ => throw IllegalStateException("Not a single element!")
 }).doSomething

Its advantage over single is that it allows you to explicitly control the behavior in exceptional case (trow an exception, return fallback value).

Alternatively you can use destructuring assignment:

val Seq(single) = desiredItem
single.doSomething

In this case you'll get MatchError if desiredItem doesn't contain exactly one element.

UPD: I looked again at your code. Destructuring assignment is the way to go for you:

val collection: Seq[SomeType]
val (Seq(desiredItem), theOthers) = collection.partition(MyFunc)

desiredItem.doSomething