proptics.syntax.IndexedFoldSyntax.scala Maven / Gradle / Ivy

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package proptics.syntax

import cats.syntax.eq._
import cats.syntax.semigroup._
import cats.{Eq, Monoid}

import proptics.internal.{Forget, Indexed}
import proptics.syntax.tuple._
import proptics.{Fold_, IndexedFold_}

trait IndexedFoldSyntax {
  implicit def indexedFoldOps[I, S, T, A](indexedFold: IndexedFold_[I, S, T, A, A]): IndexedFoldsOps[I, S, T, A] =
    IndexedFoldsOps(indexedFold)
}

final case class IndexedFoldsOps[I, S, T, A](private val indexedFold: IndexedFold_[I, S, T, A, A]) extends AnyVal {
  /** combine an index and an [[IndexedFold_]] to narrow the focus to a single element */
  def index(i: I)(implicit ev: Eq[I]): Fold_[S, T, A, A] = filterByIndex(_ === i).unIndex

  /** traverse elements of an [[IndexedFold_]] whose index satisfy a predicate applied on the index */
  def filterByIndex(predicate: I => Boolean): IndexedFold_[I, S, T, A, A] = filter(predicate compose Tuple2._2)

  /** traverse elements of an [[IndexedFold_]] whose index satisfy a predicate */
  def filter(predicate: ((A, I)) => Boolean): IndexedFold_[I, S, T, A, A] = new IndexedFold_[I, S, T, A, A] {
    def apply[R: Monoid](indexed: Indexed[Forget[R, *, *], I, A, A]): Forget[R, S, T] = {
      val runForget = indexed.runIndex.runForget

      Forget(s => indexedFold.foldLeft(s)(Monoid[R].empty)((r, pair) => if (predicate(pair)) r |+| runForget(pair) else r))
    }
  }
}