Scala generics in function return value

Question

I have these classes defined.

trait ResultTrait {
}
case class PostResult (
  @Key("_id") id: String,
  success: String,
  errors: Seq[String] = Seq.empty
) extends ResultTrait

case class PostError (
  message: String,
  errorCode: String
) extends ResultTrait

This won't compile. It gives error "Required T, but found PostResult (or PostError)".

def postLead[T <: SFDCResult](accessToken: AccessToken):
        Future[T] = {
     // depends on response from request, return PostResult or PostError
}

When you make a method generic you're saying that the caller gets to decide what T is (possibly given some constraints), not you as the implementer. — Travis Brown
– Travis Brown, Commented Apr 12, 2015 at 23:07

millhouse · Accepted Answer · 2015-04-13 06:44:44Z

1

As @Travis Brown has already stated, it looks like you're trying to express the variability of the return type (i.e. "it's either a PostResult or a PostError") through generics, when really all you need is the parent trait.

Assuming your SDFCResult was an anonymization error where you meant to use ResultTrait, I would use the following:

// Make the trait sealed so we can only have our two known implementations:
sealed trait ResultTrait {}
...
// Two subclasses as before

And then your method should just be:

def postLead(accessToken: AccessToken):Future[ResultTrait] = {
  // depends on response from request, return PostResult or PostError
}

answered Apr 13, 2015 at 6:44

millhouse

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