Getting the element from a 1-element Scala collection - scala

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

Answer 2

There's no prebuilt method in the API to do that. You can create your own method to do something similar though.
scala> def single[A](xs: List[A]) = xs match{
| case List() => None
| case x::Nil => Some(x)
| case x::xs => throw new Exception("More than one element")
| }
single: [A](xs: Seq[A])Option[A]
scala> single(List(1,2,3))
java.lang.Exception: More than one element
at .single(<console>:11)
... 33 elided
scala> single(List(1))
res13: Any = Some(1)
scala> single(List())
res14: Any = None

Answer 3

Like others indicated, there is no library implementation of what you seek. But it's easy to implement your own using a Pimp My Library approach. For example you can do the following.
object Main extends App {
object PML {
implicit class TraversableOps[T](val collection: TraversableOnce[T]) {
def single: Option[T] = collection.toList match {
case List(x) => Some(x)
case _ => None
}
}
}
import PML._
val collection: Seq[Int] = Seq(1, 2)
val (desiredItem, theOthers) = collection.partition(_ < 2)
println(desiredItem.single) // Some(1)
println(collection.single) // None
println(List.empty.single) // None
}