standard recursive filter function for blog post

filterRec.kt

fun <T> List<T>.tail() = drop(1)
fun <T> List<T>.head() = first()

fun <T> filter(l: List<T>, f: (T) -> Boolean): List<T> {
    if (l.isEmpty()) {
        return listOf()
    } else {
        return if (f(l.head())) {
            listOf(l.head()) + filter(l.tail(), f)
        } else {
            filter(l.tail(), f)
        }
    }
}