How an Array Monad could look like (naÏve approach)

CatsUtil.scala

import cats.Monad

import scala.annotation.tailrec
import scala.collection.mutable.ArrayBuffer

object CatsUtil {

  implicit val arrayMonad: Monad[Array] = new Monad[Array] {
    override def flatMap[A, B](fa: Array[A])(f: A => Array[B]): Array[B] = {
      if (fa.isEmpty) return Array.empty
      val buffer = ArrayBuffer.empty[B]
      val iterator: Iterator[A] = fa.iterator
      while (iterator.hasNext) {
        val nextValue: A = iterator.next()
        f(nextValue).foreach(buffer.addOne)
      }
      buffer.toArray
    }

    @tailrec
    override def tailRecM[A, B](a: A)(f: A => Array[Either[A, B]]): Array[B] = f(a) match {
      case Array(Left(a1)) => tailRecM(a1)(f)
      case Array(Right(b1)) => pure(b1)
    }

    override def pure[A](a: A): Array[A] = Array(a)
  }
}