groupingBy
Creates a Grouping source from a char sequence to be used later with one of group-and-fold operations using the specified keySelector function to extract a key from each character.
inline fun <K> CharSequence.groupingBy(crossinline keySelector: (Char) -> K): Grouping<Char, K>(source)
val text = "hello world"
val charCounts = text.groupingBy { it }.eachCount()
println(charCounts) // {h=1, e=1, l=3, o=2, =1, w=1, r=1, d=1}