unused local definition warning when defining focus inside a function

[jtjeferreira]

case class Lecturer(firstName: String, lastName: String, salary: Int)
case class Department(budget: Int, lecturers: List[Lecturer])
case class University(name: String, departments: Map[String, Department])

val uni = University("oxford", Map(
  "Computer Science" -> Department(45, List(
    Lecturer("john"  , "doe", 10),
    Lecturer("robert", "johnson", 16)
  )),
  "History" -> Department(30, List(
    Lecturer("arnold", "stones", 20)
  ))
))

import monocle.Focus
import monocle.syntax.all._

val departmentsWorks = Focus[University](_.departments)
departmentsWorks.at("History").replace(None)(uni)

def foo() = {

  //warning unused local definition
  val departments = Focus[University](_.departments)
  departments.at("History").replace(None)(uni)
}

foo()

Scastie: https://scastie.scala-lang.org/TfVaZDtKSmKGrgQuI7N3Rg

versions:
scala 3.5.2
monocle 3.3.0

[jtjeferreira]

I think this is a scala compiler issue already reported in scala/scala3#17753

[som-snytt]

The fix to the warning ignores the "inline proxy", but note that it is unused:

[info]           val departmentsWorks:
[info]
[info]               monocle.PLens[University, University, Map[String, Department],
[info]                 Map[String, Department]]
[info]
[info]            =
[info]             {
[info]               val MkFocus_this: monocle.Focus.MkFocus[University] =
[info]                 monocle.Focus.apply[University]
[info]               monocle.Iso.id[University].andThen[Map[String, Department],
[info]                 Map[String, Department]](
[info]                 monocle.Lens.apply[University, Map[String, Department]]((
[info]                   from: University) => from.departments)((
[info]                   to: Map[String, Department]) => (from: University) =>
[info]                   from.copy(from.copy$default$1, departments = to))
[info]               )
[info]             }