Scala: elegant way to use require to validate multiple options?

Question

I have two solutions for this problem. I do not like both of them so I was wondering if there's a more elegant solution.

import java.util.Date
import scala.math.Ordered.orderingToOrdered

// Solution # 1:
case class A(startDate: Option[Date] = None,
             endDate: Option[Date]   = None) {
  require(if (startDate.isEmpty && endDate.isEmpty) false else true, 
          "Either startDate or endDate must be defined")
  require(if (startDate.isDefined && endDate.isDefined) startDate.get < endDate.get else true,  
          s"startDate:${startDate.get} must be less than endDate:${endDate.get}")
  // Problem: multiple checks using isEmpty and isDefined followed by .get
}

// Solution # 2:
case class B(startDate: Option[Date] = None,
             endDate: Option[Date]   = None) {
  val (requirement, msg) = (startDate, endDate) match {
    case (None, None)                  => false -> "Either startDate or endDate must be defined"
    case (Some(s), Some(e)) if (s > e) => false -> s"startDate:$s must be less than endDate:$e"
    case _                             => true  -> "OK" // Problem: redundant statement
  }
  require(requirement, msg)
}

Conditions:

1. Both startDate or endDate can be None
2. startDate cannot be greater than endDate

Answer 1

Just a little craigslist ad for ensuring:

scala> val a = Option(2); val b = Option(3)
a: Option[Int] = Some(2)
b: Option[Int] = Some(3)

scala> (for (x <- a; y <- b) yield { require(x < y); y - x }).ensuring(_.nonEmpty)
res0: Option[Int] = Some(1)

scala> val a = Option(42)
a: Option[Int] = Some(42)

scala> (for (x <- a; y <- b) yield { require(x < y); y - x }).ensuring(_.nonEmpty)
java.lang.IllegalArgumentException: requirement failed
  at scala.Predef$.require(Predef.scala:207)
  at $anonfun$1$$anonfun$apply$1.apply$mcII$sp(<console>:10)
  at $anonfun$1$$anonfun$apply$1.apply(<console>:10)
  at $anonfun$1$$anonfun$apply$1.apply(<console>:10)
  at scala.Option.map(Option.scala:146)
  at $anonfun$1.apply(<console>:10)
  at $anonfun$1.apply(<console>:10)
  at scala.Option.flatMap(Option.scala:171)
  ... 33 elided

scala> val a: Option[Int] = None
a: Option[Int] = None

scala> (for (x <- a; y <- b) yield { require(x < y); y - x }).ensuring(_.nonEmpty)
java.lang.AssertionError: assertion failed
  at scala.Predef$.assert(Predef.scala:151)
  at scala.Predef$Ensuring$.ensuring$extension2(Predef.scala:255)
  ... 33 elided

For your use case, put your constraints in a custom apply:

scala> :pa
// Entering paste mode (ctrl-D to finish)

case class C(i: Int, j: Int)
object C {
  def apply(a: Option[Int] = None, b: Option[Int] = None) = (
    for (x <- a; y <- b) yield {
      require(x < y, "Order violation")
      new C(x, y)
    }
  ).ensuring(_.nonEmpty, "Missing value").get
}

// Exiting paste mode, now interpreting.

defined class C
defined object C

scala> C(Option(42))
java.lang.AssertionError: assertion failed: Missing value
  at scala.Predef$Ensuring$.ensuring$extension3(Predef.scala:256)
  at C$.apply(<console>:13)
  ... 33 elided

scala> C(Option(42),Option(3))
java.lang.IllegalArgumentException: requirement failed: Order violation
  at scala.Predef$.require(Predef.scala:219)
  at C$$anonfun$apply$1$$anonfun$apply$2.apply(<console>:12)
  at C$$anonfun$apply$1$$anonfun$apply$2.apply(<console>:12)
  at scala.Option.map(Option.scala:146)
  at C$$anonfun$apply$1.apply(<console>:12)
  at C$$anonfun$apply$1.apply(<console>:12)
  at scala.Option.flatMap(Option.scala:171)
  at C$.apply(<console>:12)
  ... 33 elided

scala> C(Option(2),Option(3))
res5: C = C(2,3)

Edit: only one arg is required.

scala> :pa
// Entering paste mode (ctrl-D to finish)

case class C(a: Option[Int] = None, b: Option[Int] = None) {
  require(a orElse b nonEmpty, "No value")
  for (x <- a; y <- b) require(x < y, "Order violation")
}

// Exiting paste mode, now interpreting.

warning: there was one feature warning; re-run with -feature for details
defined class C

scala> C()
java.lang.IllegalArgumentException: requirement failed: No value
  at scala.Predef$.require(Predef.scala:219)
  ... 34 elided

scala> C(Option(42))
res1: C = C(Some(42),None)

scala> C(Option(42),Option(3))
java.lang.IllegalArgumentException: requirement failed: Order violation
  at scala.Predef$.require(Predef.scala:219)
  at C$$anonfun$1$$anonfun$apply$mcVI$sp$1.apply$mcVI$sp(<console>:9)
  at C$$anonfun$1$$anonfun$apply$mcVI$sp$1.apply(<console>:9)
  at C$$anonfun$1$$anonfun$apply$mcVI$sp$1.apply(<console>:9)
  at scala.Option.foreach(Option.scala:257)
  at C$$anonfun$1.apply$mcVI$sp(<console>:9)
  at C$$anonfun$1.apply(<console>:9)
  at C$$anonfun$1.apply(<console>:9)
  at scala.Option.foreach(Option.scala:257)
  ... 34 elided

scala> C(Option(2),Option(3))
res3: C = C(Some(2),Some(3))

Answer 2

Here is how I would write it:

import java.util.Date

case class MyTest(startDate: Option[Date] = None, endDate: Option[Date] = None) {
  require(startDate.isDefined || endDate.isDefined,
    "Either startDate or endDate must be defined")
  require(!(startDate.isDefined && endDate.isDefined) || (startDate.get.before(endDate.get)),
    s"startDate: ${startDate.get} must be less than endDate:${endDate.get}")
}

object Test extends App {
  // Gives "Either startDate or endDate must be defined" as expected
  //val m1 = MyTest(None, None)

  // These run OK
  val m2 = MyTest(Some(new Date(1234)), None)
  val m3 = MyTest(None, Some(new Date(4321)))
  val m4 = MyTest(Some(new Date(1234)), Some(new Date(4321)))

  // Gives "startDate: Thu Jan 01 01:00:00 CET 1970 must be less than endDate: Thu Jan 01 01:00:00 CET 1970" as expected
  //val m4 = MyTest(Some(new Date(4321)), Some(new Date(1234)))
}

No need for these if inside the requires. Also note the second require uses "not(a) or b" is equivalent to "a => b". Last but not least, as you check for your options to be defined, then you are safe when doing the .get on your options.