collectFirst.scala

package notes

import cats.data.{OptionT, Writer}
import cats.free.Free
import cats.instances.option._
import cats.instances.stream._
import cats.instances.vector._
import cats.syntax.applicative._
import cats.syntax.coflatMap._
import cats.syntax.either._
import cats.syntax.foldable._
import cats.syntax.functor._
import cats.syntax.option._
import cats.syntax.writer._
import cats.{Foldable, Monad, _}

import scala.collection.immutable.Seq

object CollectFirst extends App {
  implicit class FoldableSyntax[F[_], A](val xs: F[A]) extends AnyVal {
    def collectFirstM[B, M[_]](f: A => M[Option[B]])(implicit M: Monad[M], F: Foldable[F]): M[Option[B]] =
      xs.foldRight(Now(OptionT.none[M, B]))((x, opt) => Eval.later(OptionT(f(x)).orElse(opt.value))).value.value

    def collectFirstSM[B, M[_]](f: A => M[Option[B]])(implicit M: Monad[M], F: Foldable[F]): M[Option[B]] =
      xs.foldRight(Now(OptionT.none[Free[M, ?], B]))((x, opt) => Eval.later(OptionT(Free.liftF(f(x))).orElse(opt.value))).value.value.runTailRec
  }

  implicit class SeqSyntax[F[_], A](val xs: Seq[A]) extends AnyVal {
    def collectFirstRecM[B, M[_]](f: A => M[Option[B]])(implicit M: Monad[M]): M[Option[B]] =
      M.tailRecM(xs) {
        case x +: rest => f(x).map(_.coflatten.toRight(rest))
        case _ => none.asRight.pure[M]
      }
  }


  def filterLog[A](f: A => Boolean)(x: A): Writer[Vector[String], Option[A]] =
    if (f(x)) Vector(s"found $x").tell as x.some
    else Vector(s"dropping $x").tell as none[A]


  println(Stream.from(2).collectFirstM(filterLog[Int](10001 % _ == 0) _).written)
  println(Stream.from(2).collectFirstSM(filterLog[Int](10009 % _ == 0) _).written)
  println(Stream.from(2).collectFirstRecM(filterLog[Int](10009 % _ == 0) _).written)
}