Martin Flower Martin Flower - 2 months ago 19
Scala Question

Scala - compare two Option[Seq[String]]

I am new to the Scala language - I seem to have a problem finding the right syntax for sorting and comparing two Option[Seq[String]]. The two should be equals irrespective of contents order. The last line in this test fails. I hope this code sufficiently documents the question.

package seqtest

import org.scalatest.{Matchers, FlatSpec}

class SortCollectionsTest extends FlatSpec with Matchers {

"Different seq order" should "not affect equality" in {

val seqAB1 = Seq("A","B")
val seqAB2 = Seq("A","B")
// two different sequences are equal
assert(seqAB1 == seqAB2)

val seqBA1 = Seq("B","A")
// two different sequences are not equal if different order
assert(seqAB1 != seqBA1)

// but is possible to convert sequence to list and sort
assert(seqAB1.toList.sorted == seqBA1.toList.sorted)


// now do the same thing with Option

val someSeqAB1 = Some(Seq("A","B"))
val someSeqAB2 = Some(Seq("A","B"))
// two different option sequences are equal
assert(someSeqAB1 == someSeqAB2)

val someSeqBA = Some(Seq("B","A"))
// two different optional sequences are not equal if different order
assert(someSeqAB1 != someSeqBA)

// Option can be converted into list (unsorted)
assert(someSeqAB1.toList != someSeqBA.toList)

// problem
// two different optional sequences cannot be sorted
// compilation error
// Error:(42, 30) No implicit Ordering defined for Seq[String].
// Error:(42, 30) not enough arguments for method sorted: (implicit ord: scala.math.Ordering[Seq[String]])List[Seq[String]].
// Unspecified value parameter ord.
assert(someSeqAB1.toList.sorted == someSeqBA.toList.sorted)
}
}

Answer

I think the much easier solution would be to map the options:

assert(someSeqAB1.map{ _.sorted } == someSeqBA.map{ _.sorted })

Btw, to the sorted version the assertion is equal.