TraversableEqualityConstraints

Provides three implicit methods that loosen the equality constraint defined by TypeCheckedTripleEquals for Scala Traversables to one that more closely matches Scala's approach to Traversable equality.

Scala's approach to Traversable equality is that if the objects being compared are ether both Seqs, both Sets, or both Maps, 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 TypeCheckedTripleEquals, because Vector and ListBuffer are not in a subtype/supertype relationship:

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.CanEqual[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 TraversableEqualityConstraint, however, the comparison will be allowed:

scala> import TraversableEqualityConstraints._
import TraversableEqualityConstraints._

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

The equality constraints provided by this trait require that left and right sides are both subclasses of either scala.collection.GenSeq, scala.collection.GenSet, or scala.collection.GenMap, and that an CanEqual can be found for the element types for Seq and Set, or the key and value types for Maps. 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.CanEqual[scala.collection.immutable.Vector[Int],
 scala.collection.mutable.ListBuffer[java.util.Date]]
             Vector(1, 2) === ListBuffer(new Date, new Date)
                          ^

This trait simply mixes together SeqEqualityConstraints, SetEqualityConstraints, and MapEqualityConstraints.