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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]
}
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