org.scalactic.SetEqualityConstraints.scala Maven / Gradle / Ivy
/*
* Copyright 2001-2013 Artima, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.scalactic
import TripleEqualsSupport._
/**
* Provides an implicit method that loosens the equality constraint defined by TypeCheckedTripleEquals or ConversionCheckedTripleEquals
* for Scala Sets to one that more closely matches Scala's approach to Set equality.
*
*
* Scala's approach to Set equality is that if both objects being compared are Sets, the elements are compared to determine equality.
* This means you could compare an immutable TreeSet and a mutable HashSet for equality, for instance, and get true so long as the
* two Sets contained the same elements in the same order. Here's an example:
*
*
*
* scala> import scala.collection.immutable.TreeSet
* import scala.collection.immutable.TreeSet
*
* scala> import scala.collection.mutable.HashSet
* import scala.collection.mutable.HashSet
*
* scala> TreeSet(1, 2) == HashSet(1, 2)
* res0: Boolean = true
*
*
*
* Such a comparison would not, however, compile if you used === under either TypeCheckedTripleEquals or ConversionCheckedTripleEquals,
* because TreeSet and HashSet 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> TreeSet(1, 2) === HashSet(1, 2)
* <console>:16: error: types scala.collection.immutable.TreeSet[Int] and
* scala.collection.mutable.HashSet[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.TreeSet[Int],
* scala.collection.mutable.HashSet[Int]]
* TreeSet(1, 2) === HashSet(1, 2)
* ^
*
*
*
* If you mix or import the implicit conversion provided by SetEqualityConstraint, however, the comparison will be allowed:
*
*
*
* scala> import SetEqualityConstraints._
* import SetEqualityConstraints._
*
* scala> TreeSet(1, 2) === HashSet(1, 2)
* res2: Boolean = true
*
*
*
* The equality constraint provided by this trait requires that both left and right sides are subclasses of scala.collection.GenSet and that
* an EqualityConstraint can be found for the element types. In the example above, both the TreeSet and
* HashSet are subclasses of scala.collection.GenSet, and the regular TypeCheckedTripleEquals provides equality
* constraints for the element types, both of which are Int. By contrast, this
* trait would not allow a TreeSet[Int] to be compared against a HashSet[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> TreeSet(1, 2) === HashSet(new Date, new Date)
* <console>:20: error: types scala.collection.immutable.TreeSet[Int] and
* scala.collection.mutable.HashSet[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.TreeSet[Int],
* scala.collection.mutable.HashSet[java.util.Date]]
* TreeSet(1, 2) === HashSet(new Date, new Date)
* ^
*
*
* @author Bill Venners
*/
trait SetEqualityConstraints {
import scala.language.higherKinds
/**
* Provides an equality constraint that allows two subtypes of scala.collection.GenSets to be compared for equality with === so long
* as an EqualityConstraint is available for the element types.
*/
implicit def setEqualityConstraint[EA, CA[ea] <: collection.GenSet[ea], EB, CB[eb] <: collection.GenSet[eb]](implicit equalityOfA: Equality[CA[EA]], ev: EA CanEqual EB): CA[EA] CanEqual CB[EB] = new EqualityConstraint[CA[EA], CB[EB]](equalityOfA)
}
/**
* Companion object that facilitates the importing of SetEqualityConstraints members as
* an alternative to mixing it in. One use case is to import SetEqualityConstraints members so you can use
* them in the Scala interpreter.
*/
object SetEqualityConstraints extends SetEqualityConstraints