How to get the values from an extended class in Scala? - scala

Question

I am creating a command-line app where it reads what the user puts in and then makes a POST request with that data (I've just started).
My problem is I've never seen a class extended like this before and it's throwing me off. How do I get the values from CommandApp so that I can use it in my Main Object?
import cats.implicits._
import com.monovore.decline._
object Main extends CommandApp(name = "hello-world",
header = "Says hello!",
main = {
val userOpt = Opts.option[String]("target", help = "Person to greet.").withDefault("world")
val quietOpt = Opts.flag("quiet", help = "Whether to be quiet.").orFalse
(userOpt, quietOpt).mapN { (user, quiet) =>
if (quiet) println("...")
else println(s"Hello $user!")
}
}) {
// How do I get the value from user into here.
// Eventually I will want to use that user data variable for a POST request.
}

Answer 1

If I understood you correctly you want to pattern match the case class Parameters of CommandApp from the main Object?
In Scala this isn't directly possible since Main is a child of CommandApp and not the other way round (you can only pattern match to child case classes). But you can use traits:
Declare a new Trait that has no direct function
CommandApp has to inherit that trait
Convert your main Object to that trait type
Pattern matching
Since i don't have your class definitions, i wrote a small example. Let's say your class looks like this:
case class Test(str:String)
class MainTest extends Test("Hello World!")
and you want to access the str value of Test like this:
val t = new MainTest()
t match{
case Test(str) => println(str)
case _ => println("Could not match")
}
This would not work since MainTest is not a case class and therefore cannot be pattern matched. But if you declare a new trait which Test inherits:
trait InheritanceTrait
case class Test(str:String) extends InheritanceTrait
class MainTest extends Test("Hello World!")
You can pattern match a MainTest object by casting it to a InheritanceTrait, which has a pattern-matching child class (therefore can be pattern matched):
val t = new MainTest()
t.asInstanceOf[InheritanceTrait] match{
case Test(str) => println(str)
case _ => println("Could not match")
} //prints "Hello World!"
If you don't want to or simply can't declare a super class of CommandApp you could use a dirty alternative by converting t to the Any type, since every class inherits from Any. Would work just as fine.
My example then would like this:
object Main{
case class Test(str:String)
class MainTest extends Test("Hello World!")
def main(args:Array[String]){
val t = new MainTest()
t.asInstanceOf[Any] match{
case Test(str) => println(str)
case _ => println("Could not match")
}
}
}
I hope this solved your problem!

Answer 2

It should be noted that CommandApp isn't a case class, it's a regular abstract class.
The intended use of CommandApp appears to be that your entire program goes into main and when main completes, your program is done. This is evidenced by the class having a name ending in App (c.f. IOApp) and has main.
So what you would do is define a function elsewhere which takes whatever values you've validated that you want to use (e.g user in this case) as arguments and does stuff with them, e.g.
// Disclaimer: I'm guessing from the code that userOpt is Option[String]
def sendUserAsPostRequest(user: String): Unit = {
// sending the user as a POST is left as an exercise for the reader
}
You can place that function in Main or in a different object (or in a different class as long as your main constructs an instance of that class). You're main would then look like (I'm assuming that for this, you're not interested in quiet):
main = {
val userOpt = Opts.option[String]("target", help = "Person to greet.").withDefault("world")
userOpt.map(sendUserAsPostRequest)
}