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Defer.scala
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Defer.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
import scala.util.control.TailCalls.TailRec
/**
* Defer is a type class that shows the ability to defer creation
* inside of the type constructor F[_].
*
* This comes up with F[_] types that are implemented with a trampoline
* or are based on function application.
*
* The law is that defer(fa) is equivalent to fa, but not evaluated immediately,
* so
* {{{
* scala> import cats._
* scala> import cats.syntax.all._
*
* scala> var evaluated = false
* scala> val dfa = Defer[Eval].defer {
* | evaluated = true
* | Eval.now(21)
* | }
*
* scala> evaluated
* res0: Boolean = false
*
* scala> Eq[Eval[Int]].eqv(dfa, Eval.now(21))
* res1: Boolean = true
* }}}
*/
trait Defer[F[_]] extends Serializable {
def defer[A](fa: => F[A]): F[A]
/**
* Defer instances, like functions, parsers, generators, IO, etc...
* often are used in recursive settings where this function is useful
*
* fix(fn) == fn(fix(fn))
*
* example:
*
* val parser: P[Int] =
* Defer[P].fix[Int] { rec =>
* CharsIn("0123456789") | P("(") ~ rec ~ P(")")
* }
*
* Note, fn may not yield a terminating value in which case both
* of the above F[A] run forever.
*/
def fix[A](fn: F[A] => F[A]): F[A] = {
lazy val res: F[A] = fn(defer(res))
res
}
}
object Defer {
def apply[F[_]](implicit defer: Defer[F]): Defer[F] = defer
implicit def catsDeferForEq: Defer[Eq] = cats.implicits.catsDeferForEq
implicit def catsDeferForEquiv: Defer[Equiv] = cats.implicits.catsDeferForEquiv
implicit def catsDeferForFunction0: Defer[Function0] = cats.instances.function.catsSddDeferForFunction0
implicit def catsDeferForFunction1[A]: Defer[Function1[A, *]] = cats.instances.function.catsStdDeferForFunction1[A]
implicit def catsDeferForHash: Defer[Hash] = cats.implicits.catsDeferForHash
implicit def catsDeferForOrder: Defer[Order] = cats.instances.order.catsDeferForOrder
implicit def catsStdDeferForOrdering: Defer[Ordering] = cats.instances.ordering.catsStdDeferForOrdering
implicit def catsDeferForPartialOrder: Defer[PartialOrder] = cats.instances.partialOrder.catsDeferForPartialOrder
implicit def catsStdDeferForPartialOrdering: Defer[PartialOrdering] =
cats.instances.partialOrdering.catsStdDeferForPartialOrdering
implicit def catsDeferForShow: Defer[Show] = cats.implicits.catsDeferForShow
implicit def catsDeferForTailRec: Defer[TailRec] = cats.instances.tailRec.catsInstancesForTailRec
}