What stdlib function turns

foo

into

bar

?

val foo = listOf(
  'a' to 1,
  'a' to 2,
  'b' to 3
)

val bar = mapOf(
  'a' to listOf(1, 2),
  'b' to listOf(3)
)

UPDATE: I found the answer

fun <T, K, V> Iterable<T>.groupBy(
    keySelector: (T) -> K,
    valueTransform: (T) -> V
): Map<K, List<V>>

foo.associateBy{} ?

that returns

Map<Char, Int>

instead of the desired

Map<Char, List<Int>>

foo.groupBy { it.first }

?

that returns

Map<Char, List<Pair<Char, Int>>>

nice!