[ptx]

They are similar but slightly different. "single" checks that there is exactly one matching element:

  >>> listOf(1, 2, 3).single { it > 2 }
  res0: kotlin.Int = 3

  >>> listOf(1, 2, 3).single { it > 1 }
  java.lang.IllegalArgumentException: Collection contains more than one matching element.

  >>> listOf(1, 2, 3).single { it > 100 }
  java.util.NoSuchElementException: Collection contains no element matching the predicate.

Whereas "firstOrNull" returns the first result even if there are more, or null if there are none:

  >>> listOf(1, 2, 3).firstOrNull { it > 2 }
  res0: kotlin.Int? = 3

  >>> listOf(1, 2, 3).firstOrNull { it > 1 }
  res1: kotlin.Int? = 2

  >>> listOf(1, 2, 3).firstOrNull { it > 100 }
  res2: kotlin.Int? = null

Python's "next" function kind of acts like "single", except that it doesn't check that there aren't more than one matching element. I wish the itertools module had a function that did this.