path: root/examples/shared/src/main/scala/eq.scala

/* Magnolia, version 0.7.1. Copyright 2018 Jon Pretty, Propensive Ltd.
 *
 * The primary distribution site is: http://co.ntextu.al/
 *
 * 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 magnolia.examples

import magnolia._
import scala.language.experimental.macros

/** typeclass for testing the equality of two values of the same type */
trait Eq[T] { def equal(value: T, value2: T): Boolean }

/** companion object to [[Eq]] */
object Eq {

  /** type constructor for the equality typeclass */
  type Typeclass[T] = Eq[T]

  /** defines equality for this case class in terms of equality for all its parameters */
  def combine[T](ctx: CaseClass[Eq, T]): Eq[T] = new Eq[T] {
    def equal(value1: T, value2: T) = ctx.parameters.forall { param =>
      param.typeclass.equal(param.dereference(value1), param.dereference(value2))
    }
  }

  /** choose which equality subtype to defer to
    *
    *  Note that in addition to dispatching based on the type of the first parameter to the `equal`
    *  method, we check that the second parameter is the same type. */
  def dispatch[T](ctx: SealedTrait[Eq, T]): Eq[T] = new Eq[T] {
    def equal(value1: T, value2: T): Boolean = ctx.dispatch(value1) {
      case sub =>
        sub.cast.isDefinedAt(value2) && sub.typeclass.equal(sub.cast(value1), sub.cast(value2))
    }
  }

  /** equality typeclass instance for strings */
  implicit val string: Eq[String] = new Eq[String] { def equal(v1: String, v2: String) = v1 == v2 }

  /** equality typeclass instance for integers */
  implicit val int: Eq[Int] = new Eq[Int] { def equal(v1: Int, v2: Int) = v1 == v2 }

  /** binds the Magnolia macro to the `gen` method */
  implicit def gen[T]: Eq[T] = macro Magnolia.gen[T]
}