The below code is not good

fun process(stringList: List<String>?, removeString: String): Int? = stringList
        ?.filterNot { it == removeString }
        ?.map { it.length }
        ?.sum()

as it introduces a lots of unnecessary branches. Try create unit test for it, and see if you could easily achieve 100% branch coverage. You’ll understand what I meant. Optionally you could look at the decompile code, and see the longer code generated compare to using let to eliminate the extra ?.