Difference between object with main() and extends App in scala - scala

Question

I'm working through ScalaInAction (book is still a MEAP, but code is public on github)
Right now I'm in chapter 2 looking at this restClient: : https://github.com/nraychaudhuri/scalainaction/blob/master/chap02/RestClient.scala
First, I setup intelliJ with scala extensions and created a HelloWorld with main():
<ALL the imports>
object HelloWorld {
def main(args: Array[String]) {
<ALL the rest code from RestClient.scala>
}
}
I get the following error when compiling:
scala: forward reference extends over defintion of value command
val httppost = new HttpPost(url)
^
I can fix this by moving the following lines around until the ordering is correct with relation to the def's
require( args.size >= 2, "You need at least two arguments to make a get, post, or delete request")
val command = args.head
val params = parseArgs(args)
val url = args.last
command match {
case "post" => handlePostRequest
case "get" => handleGetRequest
case "delete" => handleDeleteRequest
case "options" => handleOptionsRequest
}
While browsing the github page, I found this: https://github.com/nraychaudhuri/scalainaction/tree/master/chap02/restclient
Which uses implements RestClient.scala using extends App instead of a main() method:
<All the imports>
object RestClient extends App {
<All the rest of the code from RestClient.scala>
}
I then changed my object HelloWorld to just use extends App instead of implementing a main() method and it works without errors
Why does the main() method way of doing this generate the error but the extends App does not?

Answer 1

Because main() is a method, and variable's in method could not be forward reference.
For example:
object Test {
// x, y is object's instance variable, it could be forward referenced
def sum = x + y // This is ok
val y = 10
val x = 10
}
But code in a method could not forward referenced.
object Test {
def sum = {
val t = x + y // This is not ok, you don't have x, y at this point
val x = 10
val y = 20
val z = x + y // This is ok
}
}
In your case, if you copy paste all codes from RestClient.scala to main(), you will have the same issue because var url is declared after its usage in handlePostRequest.