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eq.scala
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eq.scala
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/*
* Copyright (c) 2015 Typelevel
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
* the Software, and to permit persons to whom the Software is furnished to do so,
* subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
* FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
* COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
* IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
package cats
package instances
import scala.annotation.tailrec
trait EqInstances extends kernel.instances.EqInstances {
implicit val catsContravariantMonoidalForEq: ContravariantMonoidal[Eq] =
new ContravariantMonoidal[Eq] {
/**
* Defaults to the trivial equivalence relation
* contracting the type to a point
*/
def unit: Eq[Unit] = Eq.allEqual
/**
* Derive an `Eq` for `B` given an `Eq[A]` and a function `B => A`.
*
* Note: resulting instances are law-abiding only when the functions used are injective (represent a one-to-one mapping)
*/
def contramap[A, B](fa: Eq[A])(f: B => A): Eq[B] =
Eq.by(f)(fa)
def product[A, B](fa: Eq[A], fb: Eq[B]): Eq[(A, B)] =
(left, right) => fa.eqv(left._1, right._1) && fb.eqv(left._2, right._2)
}
implicit def catsDeferForEq: Defer[Eq] = EqInstances.catsDeferForEqCache
}
object EqInstances {
private val catsDeferForEqCache: Defer[Eq] =
new Defer[Eq] {
case class Deferred[A](fa: () => Eq[A]) extends Eq[A] {
private lazy val resolved: Eq[A] = {
@tailrec
def loop(f: () => Eq[A]): Eq[A] =
f() match {
case Deferred(f) => loop(f)
case next => next
}
loop(fa)
}
override def eqv(x: A, y: A): Boolean = resolved.eqv(x, y)
}
override def defer[A](fa: => Eq[A]): Eq[A] = {
lazy val cachedFa = fa
Deferred(() => cachedFa)
}
}
}