Just found I’ve made a completely invalid assumption about `singleOrNull` - I assumed it would throw...

# announcements

[Rob Elliot]

Just found I’ve made a completely invalid assumption about singleOrNull - I assumed it would throw an exception if size > 1, but it returns null.

[Rob Elliot]

I’d quite like something that asserts an Iterable<T> has 0..1 elements by returning a T? or throwing an exception if size > 1.

[Rob Elliot]

(what’s the term for the types of numbers in a collection? 0, 1, 0..1, 0..*, 1..* - it’s not arity…)

[Timo Gruen]

Shouldn’t you used Optional instead if you got a construct like that?

[Timo Gruen]

Normally I’m choosing first() for sth. like that. Or i’m simply asserting that the collection size is less-equal 1. But don’t quite understand the reason to choose Collection if you are just having zero or exactly one value(s)?

[raulraja]

@Rob Elliot this function may do what you want:

fun <A> Iterable<A>.firstOrThrow(): A =
  if (size > 1) throw IllegalStateException("unexpected size == $size. $this")
  else first()

but as @Timo Gruen said maybe if you have a data structure that can only contain a single potentially absent element then nullable types it’s a better option.

[Rob Elliot]

I’m filtering an Iterable to one or zero.

[Youssef Shoaib [MOD]]

To add to raulraja's suggestion this possibly fits better with your usecase:

fun <A> Iterable<A>.singleOrThrow(): A? =
  if (size > 1) throw IllegalStateException("unexpected size == $size. $this")
  else firstOrNull() // Returns A if size == 1 and null if size == 0

[Rob Elliot]

Thanks all - it’s not a problem with writing the function, just surprise at the original one’s behaviour (though as I think about it, it is consistent with other xOrNull behaviour).

(I’s the singleOrNull(predicate: (T) -> Boolean) that I was using btw)