What does "Forward reference extends over definition of value" mean in Scala?

Question

I keep getting

Forward reference extends over definition of value a

error while trying to compile my application (inside SBT).

a is just val a = "" , the error is triggered by accessing a particular parameter (of the function) right before a definition. The parameter is of a simple case class type with all three fields of Option[...] type (2 of Option[org.joda.time.DateTime] and 1 of Option of an enumeration value).

What can "Forward reference extends over definition of value" mean at all and what can be the ways to fight it?

Answer 1

The error message means that you have a forward reference to a method, ie you're calling a method before you define it, and that the definition of the value x appears between that forward reference and the definition of the method. It is only legal to have forward references if there are no value definition between the reference and the referred method definiton.

Answer 2

Basically it's a bug.

The fix is to declare a method before calling it. I don't know why.

def a(input: String){

}

val k = a("ravi")

Answer 3

Depending on your scalac version, there have been bugs where synthetic methods cause this error.

https://issues.scala-lang.org/browse/SI-6278

Illustration, imagine f is generated:

object Test {
  def main(args: Array[String]) {
    class NotUsed {val x = f}
    val dummy = false
    def f = true
  }
}

Case classes, default arguments and implicit classes involve synthetics.

In the example code from that ticket (which has been fixed), you can break the ok method by moving the implicit to the end of the function:

object tiny {

  def main(args: Array[String]) {
    ok(); nope()
  }
  def ok() {
    class Foo(val i: Int) {
      def foo[A](body: =>A): A = body
    }
    implicit def toFoo(i: Int): Foo = new Foo(i)

    val k = 1
    k foo println("k?")
    val j = 2
  }
  def nope() {
    implicit class Foo(val i: Int) {
      def foo[A](body: =>A): A = body
    }

    val k = 1
    k foo println("k?")
    //lazy
    val j = 2
  }
}

what can be the ways to fight it?

As implied by the comment in the code, making the definition lazy is a workaround.

Illustration 2, imagine the function is so long that you don't notice the naming problem:

object Test {
  def main(args: Array[String]) {
    class NotUsed {val xs = args}
    val dummy = false
    // oops, shadows the parameter
    def args = Seq("a","b","c")
  }
}

Answer 4

If it is referenced somewhere and scalac is confused with the sequence, making it lazy could do the trick

I guess it might be to late for an answer, and since I cannot see what you are actually trying to do I am not sure if that solves it though.

Answer 5

An example for the answer given by sepp2k

object ForwardReferenceTest {

  def main(args: Array[String]): Unit = {
    test
    val x = 1
    def test = println("hi")
  }
}

you will get error

Error:(7, 5) forward reference extends over definition of value x
    test
    ^

the function test is defined after the call and there is a value definition of x in between. Removing/Moving val x definition will solve the problem.

Answer 6

The message means that at run time scala was not able to find the reference of the method that is called in your method. This usually happens when you try to cal a method and the implementation of the called method is after the calling method.

like for ex:

implicit val userDetails: Reads[UserDetails] = (
          (JsPath \ "name").read[String] and
            (JsPath \ "providers").read[List[Provider]]
          )(UserDetails.apply _) 

implicit val providers: Reads[Provider] = (
          (JsPath \ "providerName").read[String] and
            (JsPath \ "providerUserId").read[String] and
            (JsPath \ "authtoken").read[String]
          )(Provider.apply _)

and the correct way to it.

implicit val providers: Reads[Provider] = (
      (JsPath \ "providerName").read[String] and
        (JsPath \ "providerUserId").read[String] and
        (JsPath \ "authtoken").read[String]
      )(Provider.apply _)

     implicit val userDetails: Reads[UserDetails] = (
      (JsPath \ "name").read[String] and
        (JsPath \ "providers").read[List[Provider]]
      )(UserDetails.apply _)

Answer 7

You Should check your imports. It must be importing variable name from the import, The variable name must be used in some library which is imported to your code. Remove the import.

Answer 8

In my case it was enough to move val definition after the implicit val definition, not clear why. Method that was assigned to the val definition was actually defined before the methd where it was actually called.