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ScalaUtils is an open-source library for Scala projects that contains portions of the ScalaTest project that may make sense to use in production.
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/*
 * 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.scalautils

import annotation.implicitNotFound

// I'm unaware of any reason this needs to be sealed, but can start out that way and loosen it if someone
// wants to create some other kind of constraint.

/**
 * Abstract class used to enforce type constraints for equality checks.
 */
@implicitNotFound(msg = "types ${A} and ${B} do not adhere to the equality constraint selected for the === and !== operators; the missing implicit parameter is of type org.scalautils.EqualityConstraint[${A},${B}]")
sealed abstract class EqualityConstraint[A, B] {

  /**
   * Indicates whether the objects passed as a and b are equal.
   *
   * @param a a left-hand-side object being compared with another (right-hand-side one) for equality (e.g., a == b)
   * @param b a right-hand-side object being compared with another (left-hand-side one) for equality (e.g., a == b)
   */
  def areEqual(a: A, b: B): Boolean
}

/**
 * An implementation of EqualityConstraint for two types A and B that requires an Equality[A] to
 * which its areEqual method can delegate an equality comparison.
 *
 * @param equalityofA an Equality type class for A
 */
final class BasicEqualityConstraint[A, B](equalityOfA: Equality[A]) extends EqualityConstraint[A, B] {

  /**
   * Indicates whether the objects passed as a and b are equal by returning the
   * result of invoking areEqual(a, b) on the passed equalityOfA object.
   *
   * @param a a left-hand-side object being compared with another (right-hand-side one) for equality (e.g., a == b)
   * @param b a right-hand-side object being compared with another (left-hand-side one) for equality (e.g., a == b)
   */
  def areEqual(a: A, b: B): Boolean = equalityOfA.areEqual(a, b)
}

/**
 * An implementation of EqualityConstraint for two types A and B that requires an Equality[A]
 * and a conversion function from B to A. 
 *
 * @param equalityofA an Equality type class for A
 */
final class BToAEqualityConstraint[A, B](equalityOfA: Equality[A], cnv: B => A) extends EqualityConstraint[A, B] {

  /**
   * Indicates whether the objects passed as a and b are equal by returning the
   * result of invoking areEqual(a, cnv(b)) on the passed equalityOfA object.
   *
   * 
   * In other words, the b object of type B is first converted to an A via the passed conversion
   * function, cnv, then compared for equality with the a object.
   * 
   *
   * @param a a left-hand-side object being compared with another (right-hand-side one) for equality (e.g., a == b)
   * @param b a right-hand-side object being compared with another (left-hand-side one) for equality (e.g., a == b)
   */
  override def areEqual(a: A, b: B): Boolean = equalityOfA.areEqual(a, cnv(b))
}

/**
 * An implementation of EqualityConstraint for two types A and B that requires an Equality[B]
 * and a conversion function from A to B. 
 *
 * @param equalityofB an Equality type class for B
 */
final class AToBEqualityConstraint[A, B](equalityOfB: Equality[B], cnv: A => B) extends EqualityConstraint[A, B] {

  /**
   * Indicates whether the objects passed as a and b are equal by return the
   * result of invoking areEqual(cnv(a), b) on the passed equalityOfB object.
   *
   * 
   * In other words, the a object of type A is first converted to a B via the passed conversion
   * function, cnv, then compared for equality with the b object.
   * 
   *
   * @param a a left-hand-side object being compared with another (right-hand-side one) for equality (e.g., a == b)
   * @param b a right-hand-side object being compared with another (left-hand-side one) for equality (e.g., a == b)
   */
  override def areEqual(a: A, b: B): Boolean = equalityOfB.areEqual(cnv(a), b)
}