Kotlin: How to use higher order functions with OpenEndRange<LocalDate>

Question

I want to use higher order functions like map for open end ranges.

val from = LocalDate.now().minusDays(10)
val to = LocalDate.now()
(from ..< to).forEach(::println)

I tried to copy an example for ClosedRange<LocalDate> but it does not work.

package de.otto.di.extensions

import java.time.LocalDate

class OpenEndRangeLocalDateIterator(
    startDate: LocalDate,
    private val endExclusive: LocalDate,
    private val stepDays: Long
) : Iterator<LocalDate> {

  private var currentDate = startDate

  override fun hasNext() = currentDate.plusDays(stepDays) <= endExclusive

  override fun next(): LocalDate {
    val next = currentDate
    currentDate = currentDate.plusDays(stepDays)
    return next
  }
}

@OptIn(ExperimentalStdlibApi::class)
class OpenEndLocalDateRange(
    override val start: LocalDate,
    override val endExclusive: LocalDate,
    private val stepDays: Long = 1
) : Iterable<LocalDate>, OpenEndRange<LocalDate> {

  override fun iterator(): Iterator<LocalDate> =
      OpenEndRangeLocalDateIterator(start, endExclusive, stepDays)

  infix fun step(days: Long) = OpenEndLocalDateRange(start, endExclusive, days)
}

infix operator fun LocalDate.rangeUntil(to: LocalDate): OpenEndLocalDateRange =
   OpenEndLocalDateRange(this, to)

It is implemented for Int so I assume it must be possible somehow. How can I achieve this?

Answer 1

The issue here is that you've defined the operator function to return OpenEndRange<LocalDate> rather than OpenEndedLocalDateRange. If you change the return type of your operator function that should fix the issue.

The reason why it isn't working as is is because OpenEndRange doesn't have the higher order functions defined for it (ClosedRange doesn't have them defined as well). Int has it because the operators return an IntRange which indirectly extends Iterable<Int> via IntProgression and Iterable has these higher order functions defined, so, the only missing piece is failing to return the correct type from your operator function.