How to unionize error types in Scala?

Question

Suppose I have a few functions

val f1: Int => Error1 \/ Int = ??? // maybe should use Kleisli instead
val f2: Int => Error2 \/ Int = ???
val f3: Int => Error3 \/ Int = ???

... and I need to compose them:

val f123 = {a: Int =>  
  for(b <- f1(a); c <- f2(b); d <- f3(c)) yield d  // does not compile
}

f123 should return Error1 | Error2 | Error3 \/ Int but I don't have | in Scala and hence I am using inheritance:

sealed trait Error123
object Error1 extends Error123
object Error2 extends Error123
object Error3 extends Error123

val f123: Int => Error123 \/ Int = ... // now it compiles

... but what if I need to compose f1 and f2 ?

Should I create a new trait Error12 and make Error1 and Error2 extend it ? What if I have five functions f1, f2, ..., f4, f5 and five error classes ?

I don't like this inheritance approach because it changes already existing types Error1, Error2 et al just to define their unions. So I am wondering how to unionize these types in Scala.

Answer 1

IMHO, this is the best approach. What are you trying to do? Define a function that may return either a result or an error of type Error123, what else it there to say?

By defining Error123 as a sealed trait and extending it with N case classes better describing the error you make sure that the caller will have to handle all cases since the compiler will ask him to use an exhaustive pattern matching.

If it is that verbose and cumbersome maybe you need to rethink the ADT of your error cases.