How to transform Option[A] to Either[String, B] in Scala with Cats?

Question

I create small code samples where want to show how cats library work. While I was working on the last one example, I noticed that it probably could be more elegant:

import cats.effect.IO
import scala.collection.mutable.HashMap

val storage = HashMap[Int, String]().empty

override def deleteWord(id: Int): IO[Either[String, Unit]] =
  for {
    removedWord <- IO(storage.remove(id))
    result <- IO {
                removedWord.flatMap(_ => Some(())).toRight(s"Word with $id not found")
              }
  } yield result

What is a way to rewrite the code snippet in a more concise form using cats syntax?

Answer 1

You don't need to create another IO, as the expression in yield result will be already wrapped by IO by the for comprehension.

def deleteWord(id: Int): IO[Either[String, Unit]] =
  for {
    removedWord <- IO(storage.remove(id))
    result = removedWord.map(_=>()).toRight(s"Word with $id not found")
  } yield result

Or even

def deleteWord(id: Int): IO[Either[String, Unit]] =
  for (removedWord <- IO(storage.remove(id)))
    yield removedWord.map(_=>()).toRight(s"Word with $id not found")

Answer 2

Maybe you oversimplified your sample, but Cats will not improve such transformation

import scala.collection.mutable.HashMap
import cats.implicits._

val storage = HashMap(1 -> "abc")

def deleteWord(id: Int) =
  storage
    .remove(id)
    .fold(s"Word with $id not found".asLeft[Unit])(_ => ().asRight)

deleteWord(1)
// Either[String, Unit] = Right(())
deleteWord(2)
// Either[String, Unit] = Left("Word with 2 not found")

Answer 3

One of the solutions which I received in gitter cats chat:

import cats.implicits._

override def deleteWord(id: Int): IO[Either[String, Unit]] =
  for {
    removedWord <- IO(storage.remove(id))
    result <- IO(removedWord.toRight(s"Word with $id not found").void)
  } yield result

it seems exactly what I needed