sample-lens.scala

import  scalaz._, Scalaz._

type Field = Vector[Vector[Option[String]]]

object Field {
  def update(x: Int, y: Int): PLens[Field, Option[String]] = {
    PLens.vectorNthPLens(y) >=> PLens.vectorNthPLens(x)
  }
}

def updateDot(x: Int, y: Int, s: String)(field: Field) = {
  Field.update(x,y).set(field, Some(s))
}

val f = Vector.fill(3)(Vector.fill(3)(Option.empty[String]))

updateDot(0,0,"aaa")(f)
// => res13: Option[Field] = Some(Vector(Vector(Some(aaa), None, None), Vector(None, None, None), Vector(None, None, None)))