Scala implicit final class

Question

I'm trying to create a way to add days to a LocalDate object in Scala through implicit classes, but I continually get an error either saying "LocalDate is a final class and cannot be extended" or below:

scala:73: could not find implicit value for parameter year: Int

Here's the code I tried using:

 implicit class UVMLocalDate(val me: Int) extends AnyVal{

 implicit def days(implicit year: Int, month: Int, day: Int) {
 LocalDate.of(year,month,me + days)
 }

 }

I'm restricted to having the class behave like so:

(LocalDate.of(2015, 4, 14)) == (LocalDate.of(2015, 4, 12) + 2.days)

Example use:

val dateEx = LocalDate.of(2015, 3, 23) + 2.days
assert(dateEx == LocalDate.of(2015, 3, 25))

Thanks

Are you using java.time? In that case, you can't add 2 LocalDates together. It also doesn't provide a Scala + method. — Teo Klestrup Röijezon
Also you are taking in 3 implicit Int arguments to your "days" method, implicit arguments are resolved by type so you would always have the same value for all 3 arguments. — Angelo Genovese
I can't find much connection between the implicit class pasted and the way you want it to behave. — pedrofurla
Please rephrase the question. Providing an example of the code which would use this new days method would be immensely helpful. — Angelo Genovese

Sascha Kolberg Sascha Kolberg · Accepted Answer · 2016-04-10T15:58:02

Sounds like you want a mixture of features from scala.concurrent.duration._ and org.joda.time._.

It does not really make sense to add a date to another date and 2.days is not really a date but rather a duration. You could go ahead and re-invent the wheel by defining a new class for it:

class Duration(millis: Long)
object Duration {
  val SecondMillis = 1000
  val MinuteMillis = SecondMillis * 60
  val HourMillis = MinuteMillis * 60
  val DayMillis = HourMillis * 24

  implicit class DurationFromDays(private val underlying: Int) extends AnyVal {
    def days: Duration = new Duration(underlying * DayMillis)
  }
}

However, you could just use the API by scala.concurrent.duration.Duration.

And then you also need to define an implicit class for adding a Duration to a LocalDate.

implicit class LocalDateOps(private val underlying: LocalDate) extends AnyVal {
  def +(duration: Duration): LocalDate = ???
}

However, there are libraries (like joda-time) where the functionality to add a duration to a date is already implemented.

Scala implicit final class

1 Answers