Global Variable in Scala

Question

I am trying to use global variable in Scala. to be accessible in the whole program .

val numMax: Int = 300 

object Foo {.. }
case class Costumer { .. }
case class Client { .. }
object main {
var lst = List[Client]
// I would like to use Client as an object .

}

I got this error :

error: missing arguments for method apply in object List; follow this method with `_' if you want to treat it as a partially applied function var lst = List[A]

How can I deal with Global Variables in Scala to be accessible in the main program . Should I use class or case class in this case ?

With your code comment, it's not entirely clear what you're trying to accomplish. Do you want a globally accessible, mutable list of Client objects? Or do you want to have a single globally accessible Client instance that you want to put in a list? — Aaron Novstrup
– Aaron Novstrup, Commented Nov 9, 2013 at 1:33

Vidya · Accepted Answer · 2013-11-08 22:44:18Z

2

This isn't a global variable thing. Rather, you want to say this:

val lst = List(client1, client2)

However, I disagree somewhat with the other answers. Scala isn't just a functional language. It is both functional (maybe not as purely as it should be if you ask the Clojure fans) and object-oriented. Therefore, your OO expertise translates perfectly.

There is nothing wrong with global variables per se. The concern is mutability. Prefer val to var as I did. Also, you need to use object for singletons rather than the static paradigm you might be used to from Java.

edited Nov 8, 2013 at 22:44

answered Nov 8, 2013 at 22:28

Vidya

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6 Comments

Yann Moisan Over a year ago

global variables makes programs more difficult to test

Vidya Over a year ago

That statement is broad and oversimplified. You need to look at the context of how it's used, the size of the program, the details of the language/platform, and so on. Try to avoid broad platitudes in software development. That said, if you can do the same thing without a global variable, then do it. But it isn't quite as black-and-white as you make it seem. Obi-Wan said it best: youtube.com/watch?v=wgpytjlW5wU

Aaron Novstrup Over a year ago

It's also somewhat oversimplified to say that the concern is just mutability. The scope of a mutable variable makes the difference between a functional program (e.g., a local variable in a referentially transparent function) and a programming nightmare (i.e., a global variable). That said, it's not even clear whether the OP meant global variable. He may have meant global value and just doesn't have the terminology yet.

Vidya Over a year ago

While interesting, those are nuances I did not address because 1) they are tangential to the OP's question, 2) this isn't the place for theoretical computer science discussions, and 3) Stack Overflow frowns upon long opinion threads. I am happy to address these points in a more appropriate venue like Twitter. I would also mention though that the original question had a var. I just really hope you aren't one of those people who fights with his girlfriend on Facebook. There is a time and place for those lengthy discussions, and it isn't an online thread.

Aaron Novstrup Over a year ago

noticed the var, but the code comment makes it sound like he's possibly trying to treat a Client as a singleton value. It's not entirely clear. Props for trying to answer anyway, though.

|

Robin Green · Accepted Answer · 2013-11-08 22:15:15Z

1

The error you quote is unrelated to your attempt to create a global variable. You have missing () after the List[Client].

If you must create a global variable, you can put it in an object like Foo and reference it from other objects using Foo.numMax if the variable is called numMax.

However, global variables are discouraged. Maybe pass the data you need into the functions that need it instead. That is the functional way.

answered Nov 8, 2013 at 22:15

Robin Green

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2 Comments

DataT Over a year ago

Can you recommend other links ! I am coming from OOP world !

ValarDohaeris-fuck-OpenAI Over a year ago

Coursera has an excellent introduction to functional programming in Scala.

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