Passing Parameters Made in a Case Class to a Trait

Question

Context

For the sake of being able to change the config for testing, I have a trait with almost all the code. Then in a case class I am trying to pass in the config. In my testing I have a testing case class.

This works (but is not the funcionality I want)

trait Greeting(val name: String) {
  def msg = s"How are you, $name"
}

class prodEnvClass extends Greeting("Production Env") {
  println(msg)
}

class testEnvClass extends Greeting("Testing Env") {
  println(msg)
}

This does not work (but is the functionality I want)

trait Greeting(val name: String) {
  def msg = s"How are you, $name"
}

class prodEnvClass extends Greeting(myEnv) {
  val myEnv = fetchProdEnv()
  println(msg)
}

class testEnvClass extends Greeting(myEnv) {
  val myEnv = fetchTestEnv()
  println(msg)
}

Question

How do I pass a parameter made inside the case class body into the trait? The code that works does not match my use case. I want to use code inside the case class to make the input to the trait.

You can't AFAIK. (I also don't see a `case class` in your code btw). How would you initialize this? You need to initialize the trait to initialize the class but you also need to initialize the class to initialize the trait ? There is a conflict right there. You should make the class final and have a companion object initialize the class as needed. It can load the required config and pass it along. You can use [ciris](https://cir.is/) to load in the corresponding environments — sinanspd, Dec 15 '20 at 05:37
Although I should note, you could have the method take the name as a parameter. That is the simple solution but that's not how I would design this — sinanspd, Dec 15 '20 at 05:43
There is a similar post that needs the same functionality for a different purpose: [Extend Java abstract class in Scala](https://stackoverflow.com/q/65276291/2359227) — Tomer Shetah, Dec 15 '20 at 07:12

score 6 · Accepted Answer · answered Dec 15 '20 at 06:49

You seem to be mixing function invocation with inheritance in a way that is not supposed to work.

For function invocation, you

create binders as a function parameters, and then
pass values as arguments,

something like this:

def f(s: String) { println(s) }
f("hello (function invocation)")

When using inheritance, you

create binders by naming abstract members, and later you
pass values by providing implementations for those abstract members

something like this:

trait C { def s: String; def m() = println(s) }
new C { val s = "hello (inheritance)"; m() }

In your code, you seem to attempt to

bind the value as constructor parameter (as you would do for functions)
pass the value myEnv by overriding it differently in two subclasses (as you would do for inheritance)

Just pick one mechanism. If you want inheritance, then do inheritance:

class MyEnv(val name: String)

trait Greeting {
  def myEnv: MyEnv
  def msg = s"hello ${myEnv.name}"
}

def fetchProdEnv() = new MyEnv("prod")
def fetchTestEnv() = new MyEnv("test")

class ProdEnvClass extends Greeting {
  val myEnv = fetchProdEnv()
  println(msg)
}

class TestEnvClass extends Greeting {
  val myEnv = fetchTestEnv()
  println(msg)
}

new ProdEnvClass() // "hello prod"
new TestEnvClass() // "hello test"

but don't try to use two halves (one half from each mechanism), they don't mix this way.

Passing Parameters Made in a Case Class to a Trait

1 Answers1