perspective.FoldableK.scala Maven / Gradle / Ivy

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package perspective

import scala.language.implicitConversions

import cats._
import cats.syntax.all._

/** A higher kinded [[Foldable]] typeclass. */
trait FoldableK[F[_[_], _]]:
  extension [A[_], C](fa: F[A, C])
    /** A higher kinded equivalent of [[Foldable.foldLeft]]. */
    def foldLeftK[B](b: B)(f: B => A :~>#: B): B

    def foldRightK[B](b: B)(f: A :~>#: (B => B)): B

    /** A higher kinded equivalent of [[Foldable.foldMap]]. */
    def foldMapK[B](f: A :~>#: B)(using B: Monoid[B]): B =
      foldLeftK(B.empty)(b => [Z] => (az: A[Z]) => b.combine(f(az)))

  /** A higher kinded equivalent of [[Foldable.toList]]. */
  extension [A, C](fa: F[Const[A], C])
    def toListK: List[A] =
      fa.foldMapK(FunctionK.liftConst(List(_: A)))
      
object FoldableK:
  given idInstanceC[A]: FoldableKC[IdFC[A]] = perspective.instances.idInstanceC[A]

type FoldableKC[F[_[_]]] = FoldableK[IgnoreC[F]]