Scala Play: reading env

// 1. reading from application.conf
import javax.inject.Inject

import play.api.Configuration
import play.api.mvc._

class TestController @Inject()(cc: ControllerComponents, appConf: Configuration) extends AbstractController(cc) {

  val facultyName: Option[String] = appConf.get[Option[String]]("students.faculty.name") // also getString() is possible

  // add to routes file: GET    /testConfig    controllers.TestController.testConfig
  def testConfig = Action {
    Ok(s"Result: $facultyName")
  }
}

// application.conf:
students {
  faculty.name = "historical"
  faculty.name = ${?FACULTY_NAME}
}

        // Output:
        // Result: Some(historical)

// 2. now set up environment variable: FACULTY_NAME=mathematics
        // Result: Some(mathematics)

// 3. Bonus! What if we use ${FACULTY_NAME} instead of ${?FACULTY_NAME}?
students {
  faculty.name = ${FACULTY_NAME}
}
        // Output with environment variable defined:
        // Result: Some(mathematics)

        // Output without environment variable:
        // Internal server error, sending 500 response
        // Configuration error: Configuration error[application.conf @ file:application.conf: 380:
        // Could not resolve substitution to a value: ${FACULTY_NAME}]
        //    at play.api.Configuration$.configError(Configuration.scala:155)
        //    at play.api.Configuration$.load(Configuration.scala:101)
        //    at play.api.Configuration$.load(Configuration.scala:109)
        //    at play.api.Configuration$.load(Configuration.scala:115)