import cats._
import cats.data._
import cats.~>
import cats.implicits._
import cats.effect._
import cats.mtl._

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

class MyProgram[F[_]: Sync, FE[_]: Monad](implicit
   FE: Raise[FE, String],
   lift: FE ~> Lambda[a => F[RejectionOr[a]]]
) {

 def run(a: String, b: String): F[RejectionOr[Int]] =
   lift(
     for {
       x <- parseNumber(a)
       b <- parseNumber(b)
     } yield x + b
   )

 private def parseNumber(in: String): FE[Int] = {
   if (in.matches("-?[0-9]+")) in.toInt.pure[FE]
   else FE.raise(show"'$in' could not be parsed as a number")
 }

}