Kotlin: How to iterate all dates within a Joda Interval?

Heavily inspired by your current solution:

fun Interval.toDateTimes() = generateSequence(start) { it.plusDays(1) }
                                                 .takeWhile(::contains) 

Usage:

interval.toDateTimes()
        .forEach { println(it) }

If you need the LocalDate you could still do the following instead:

interval.toDateTimes()
        .map(DateTime::toLocalDate)
        .forEach { println(it) }

or as an extension function to Interval again:

fun Interval.toLocalDates() = toDateTimes().map(DateTime::toLocalDate)

If you want the end date to be inclusive instead, use takeWhile { it <= end } instead.

The following extension function gives a Sequence of LocalDate objects from the given Interval, which can be used to iterate those dates.

fun Interval.toLocalDates(): Sequence<LocalDate> = generateSequence(start) { d ->
    d.plusDays(1).takeIf { it < end }
}.map(DateTime::toLocalDate)

Usage:

val interval = Interval(DateTime.now().minusDays(42), DateTime.now())
interval.toLocalDates().forEach {
    println(it)
}

In this solution, the last day, DateTime.now() is not included in the Sequence since that's how Interval is implemented as well:

"A time interval represents a period of time between two instants. Intervals are inclusive of the start instant and exclusive of the end."

If, for any reason, you want to make it include the last day, just change the takeIf condition to it <= end.

I guess if you need it more than once, it would be better to overload rangeTo operator to allow this syntax

for (i in LocalDate.now() .. LocalDate.now().plusWeeks(1)) {
    System.out.print(i) // 2018-08-30 2018-08-31 2018-09-01 
}

Here is the code for operator extension:

operator fun LocalDate.rangeTo(other: LocalDate): LocalDateRange {
    return LocalDateRange(this, other)
}

And necessary classes:

class LocalDateRange(override val start: LocalDate, override val endInclusive: LocalDate)
    : ClosedRange<LocalDate>, Iterable<LocalDate> {
    override fun iterator(): Iterator<LocalDate> {
        return DateIterator(start, endInclusive)
    }
}

class DateIterator(start: LocalDate, private val endInclusive: LocalDate)
    : Iterator<LocalDate> {

    private var current = start

    override fun hasNext(): Boolean {
        return current <= endInclusive
    }

    override fun next(): LocalDate {
        current = current.plusDays(1)
        return current
    }
}