fs2-map-accumulate-errors.scala

import cats.effect._
import cats.syntax.apply._
import fs2._
import scala.util.Random

val ioa: IO[Option[Int]] = IO(Random.nextInt(100)).flatMap { n =>
  if (n % 2 == 0) IO(println(s"$n")) *> IO.pure(Some(n))
  else IO(println(s"ERROR >> $n")) *> IO.raiseError(new Exception(s"Error $n"))
}

val stream =
  Stream
    .repeatEval(ioa.attempt)
    .take(10)
    .mapAccumulate(List.empty[String]) {
      case (acc, Left(t)) => (t.getMessage :: acc, None: Option[Int])
      case (acc, Right(x)) => (acc, x)
    }

println(stream.compile.last.unsafeRunSync)