scala pattern matching on (Try,Try)

Question

I have the the following Tuple - (t1,t2) :(Try,Try)

I want to check if both succeeded or if one of them failed, but avoid code duplication. Something like:

(t1,t2) match {
case (Success(v1),Success(v2)) => new MyClass(v1,v2)
case (Failure(e),_) | (_,Failure(e)) => println(e.getMessage)
}

of course the 2nd statement won't work since I need to give different extraction variables. but then I have to check them since I don't know which failed and actually contains Throwable. I wish Try would act like Future so it'll have Try.sequence(t1,t2).

Any idea how to make this work elegantly?

Answer 1

You can convert it to a Try[MyClass] like this:

val myclass = for {
  v1 <- t1
  v2 <- t2
} yield new MyClass(v1, v2)

If t1 failed, or both t1 and t2 failed, myclass will be a Failure with the Exception for t1. If only t2 failed, myclass will be a Failure with the Exception for t2. Otherwise, myclass will be a Success. You can then process it normally using recover or whatever.

Answer 2

You could do a tail recursive call for the (_, Failure(e)) case:

@annotation.tailrec
def apply(t1: Try[Any], t2: Try[Any]): Any =
  (t1, t2) match {
    case (Success(v1), Success(v2)) => new MyClass(v1,v2)
    case (Failure(e), _) => println(e.getMessage)
    case _ => apply(t2, t1)
  }

Cats lets you do this elegantly. For any F[_]: Traverse and G[_]: Applicative, it defines the equivalant of Future.sequence:

def sequence(fa: F[G[A]]): G[F[A]]

The library also provides out of the box instances for Try. Further reading in the traverse documentation.