SeqEqualityConstraints

Provides an implicit method that loosens the equality constraint defined by TypeCheckedTripleEquals or ConversionCheckedTripleEquals for Scala Seqs to one that more closely matches Scala's approach to Seq equality.

Scala's approach to Seq equality is that if both objects being compared are Seqs, the elements are compared to determine equality. This means you could compare an immutable Vector and a mutable ListBuffer for equality, for instance, and get true so long as the two Seqs contained the same elements in the same order. Here's an example:

scala> import scala.collection.mutable.ListBuffer
import scala.collection.mutable.ListBuffer

scala> Vector(1, 2) == ListBuffer(1, 2)
res0: Boolean = true

Such a comparison would not, however, compile if you used === under either TypeCheckedTripleEquals or ConversionCheckedTripleEquals, because Vector and ListBuffer are not in a subtype/supertype relationship, nor does an implicit conversion by default exist between them:

scala> import org.scalactic._
import org.scalactic._

scala> import TypeCheckedTripleEquals._
import TypeCheckedTripleEquals._

scala> Vector(1, 2) === ListBuffer(1, 2)
<console>:16: error: types scala.collection.immutable.Vector[Int] and
 scala.collection.mutable.ListBuffer[Int] do not adhere to the equality constraint selected for
 the === and !== operators; the missing implicit parameter is of type
 org.scalactic.EqualityConstraint[scala.collection.immutable.Vector[Int],
 scala.collection.mutable.ListBuffer[Int]]
             Vector(1, 2) === ListBuffer(1, 2)
                          ^

If you mix or import the implicit conversion provided by SeqEqualityConstraint, however, the comparison will be allowed:

scala> import SeqEqualityConstraints._
import SeqEqualityConstraints._

scala> Vector(1, 2) === ListBuffer(1, 2)
res2: Boolean = true

The equality constraint provided by this trait requires that both left and right sides are subclasses of scala.collection.GenSeq and that an EqualityConstraint can be found for the element types. In the example above, both the Vector and ListBuffer are subclasses of scala.collection.GenSeq, and the regular TypeCheckedTripleEquals provides equality constraints for the element types, both of which are Int. By contrast, this trait would not allow a Vector[Int] to be compared against a ListBuffer[java.util.Date], because no equality constraint will exist between the element types Int and Date:

scala> import java.util.Date
import java.util.Date

scala> Vector(1, 2) === ListBuffer(new Date, new Date)
<console>:20: error: types scala.collection.immutable.Vector[Int] and
 scala.collection.mutable.ListBuffer[java.util.Date] do not adhere to the equality constraint selected for
 the === and !== operators; the missing implicit parameter is of type
 org.scalactic.EqualityConstraint[scala.collection.immutable.Vector[Int],
 scala.collection.mutable.ListBuffer[java.util.Date]]
             Vector(1, 2) === ListBuffer(new Date, new Date)
                          ^