Composing Errors with Coproduct

errcompose.scala

import shapeless.{ :+:, CNil, Coproduct }
import shapeless._

val error1 = Coproduct[IllegalArgumentException :+: CNil](new IllegalArgumentException)
val error2 = Coproduct[NumberFormatException :+: CNil](new NumberFormatException)

type MyError = IllegalArgumentException :+: NumberFormatException :+: CNil

object MyErrorHandler extends Poly1 {
  implicit def caseConfigNotFoundException: Case.Aux[IllegalArgumentException, String] =
    at[IllegalArgumentException](e => "bad args")

  implicit def caseWrongConfigException: Case.Aux[NumberFormatException, String] =
    at[NumberFormatException](e => "bad num")
}

def blah(): Either[IllegalArgumentException :+: CNil, String] = Left(error1)
def blim(): Either[NumberFormatException :+: CNil, String] = Left(error2)

val a: Either[IllegalArgumentException :+: CNil, String] = blah()
val b: Either[NumberFormatException :+: CNil, String] = blim()

val c: Either[MyError, String] = for {
  a1 <- a.left.map(_.embed[MyError])
  b1 <- b.left.map(_.embed[MyError])
} yield a1 + b1

val out: String = c match {
  case Left(err) => err.fold(MyErrorHandler)
  case Right(str) => str
}

println(out)