Mixing pure and impure with Cats Effect - scala

Question

Suppose we have a two-stage pure method.
def filterPositivePure(seq: Seq[Int]): Seq[Int] =
if (seq.nonEmpty) {
val sorted = seq.sorted
sorted.filter(_ > 0)
}
else seq
We needed to log the intermediate result of the calculations or any other impure action.
def filterPositiveImpure(seq: Seq[Int]): Seq[Int] =
if (seq.nonEmpty) {
val sorted = seq.sorted
println(sorted)
sorted.filter(_ > 0)
}
else seq
Then to preserve purity, we wrapped in IO the output to the log, the rest of the calculations and the result of the entire alternative if branch.
def filterPositiveIO(seq: Seq[Int]): IO[Seq[Int]] =
if (seq.nonEmpty) {
val sorted = seq.sorted
IO(println(sorted)) *> IO(sorted.filter(_ > 0))
}
else IO(seq)
Is there a more coincise way to bring purity back?

Answer 1

I think better way is working on one action as a new effect and write separate function for logging:
import cats.syntax.flatMap._
// this three imports is just to make show is working
import cats.syntax.show._
import cats.instances.list._
import cats.instances.int._
// now function looks more structural and without multiple using sorted list
def filterPositiveIO(seq: Seq[Int]): IO[Seq[Int]] = {
if (seq.nonEmpty)
withLogging(seq.toList.sorted).map(_.filter(_ > 0))
else
IO(seq)
}
// this function is just for making some logging effect and return lifted value into IO
def withLogging[A: Show](value: A): IO[A] = logging(value.show) >> IO(value)
// here you should make logging effect
def logging(strExpr: => String): IO[Unit] = ???