scalascala-catskleisli

How to create Kleisli with Kleisli.local (not local for transforming)

There are 2 local methods defined as:

final case class Kleisli[F[_], A, B](run: A => F[B]) { self =>
  ...
  def local[AA](f: AA => A): Kleisli[F, AA, B] =
    Kleisli(f.andThen(run))
  ...
}

and as:

sealed private[data] trait KleisliFunctions {
  ...
  def local[M[_], A, R](f: R => R)(fa: Kleisli[M, R, A]): Kleisli[M, R, A] =
    Kleisli(f.andThen(fa.run))
}

The 2nd one should be used as a factory method, to construct Kleisli.

Can you please offer any usecases to use the 2nd method, defined in KleisliFunctions, to create instance of Kleisli. With an example, if possible. Cannot get it, where this method could be useful.

Solution

  • For example

    val k: Kleisli[Option, String, Char] = Kleisli(_.headOption)

k.local[Int](_.toString): Kleisli[Option, Int, Char]  // Kleisli#local

Kleisli.local((_: String).toUpperCase)(k): Kleisli[Option, String, Char] // KleisliFunctions#local

    Object Kleisli extends trait KleisliFunctions.