The Align typeclass

align.scala

import scala.util.Try
import cats.Functor
import cats.Align
import cats.data.Ior

type Decimal = Int
type Roman = String

def f[F[_]: Align: Functor](fi: F[Decimal], fs: F[Roman]): F[String] =
  import cats.syntax.align.* // <- HERE
  import cats.syntax.functor.*

  // `align` is like a "zip on steroids"!
  // It pairs 2 effects regardless whether they are symmetric or not
  // i.e they have the same size, they are both defined, they are both successful, etc.
  //
  // The `Align` typeclass provides more interesting "zippers" like this.
  // Check them out! https://typelevel.org/cats/api/cats/Align.html
  fi.align(fs).map { ior =>
    ior match
      case Ior.Both(decimal, roman) => s"$decimal is $roman in Roman"
      case Ior.Left(decimal)        => s"$decimal has no Roman"
      case Ior.Right(roman)         => s"$roman has no Decimal"
  }

///

val decimals1 = List(1, 2, 3, 0)
val romans1 = List("I", "II", "III")
val out1 = f(decimals1, romans1)
// List[String] = List(1 is I in Roman, 2 is II in Roman, 3 is III in Roman, 0 has no Roman)

val decimals2 = List(1, 2, 3)
val romans2 = List("I", "II", "III", "IV")
val out1a = f(decimals2, romans2)
// List[String] = List(1 is I in Roman, 2 is II in Roman, 3 is III in Roman, IV has no Decimal)

val out1b = f(List.empty, List.empty)
// List[String] = List()

///

import cats.syntax.option.*

val out2 = f(0.some, None)
// Option[String] = Some(1 is I in Roman)

val out2a = f(1.some, "I".some)
// Option[String] = Some(1 is I in Roman)

val out2b = f(None, "X".some)
// Option[String] = Some(X has no Decimal)

val out2c = f(None, None)

///

type Either_[A] = Either[String, A]

val out3 = f(Right(1), Left("ERROR"))
// Either[String, String] = Right(1 has no Roman)

val out3a = f[Either_](Right(1), Right("I"))
// Either[String, String] = Right(1 is I in Roman)

val out3b = f(Left("ERROR"), Right("X"))
// Either[String, String] = Right(X has no Decimal)

val out3c = f[Either_](Left("ERROR"), Left("ERROR"))
// Either[String, String] = Left(ERROR)