Validator de CPF em Kotlin utilizando FP

CpfExt.kt

val knownInvalidCpfs = 
    (0..9)
      .map { i -> (1..11).map { i } }
      .map { it.joinToString("") }

fun isValid(document: String): Boolean {
    if (document in knownInvalidCpfs) return false
    
    val numbers = document.mapNotNull { it.digitToIntOrNull() }
    
    fun check(digit: Int) =
        numbers
            .zip((9 + digit) downTo 2)
            .sumOf { (f, s) -> f * s }
            .let { it * 10  % 11 }
            .let { if (it == 10) 0 else it } == numbers[8 + digit]
        
    return check(1) && check(2)
}

fun main() {
    isValid("32612124624").also(::println) //true
    isValid("11111111111").also(::println) //false
    isValid("84729378292").also(::println) //false
    isValid("38736526681").also(::println) //true
}