When you are coding in Scala, you create variables using the operator var, or you can use the operator val. The operator var allows you to create a mutable state, which is fine as long as you make it local, follow the CORE-FP principles and avoid a mutable shared state.
We will see how to use var in Scala REPL as follows:
$ scala
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_77).
Type in expressions for evaluation. Or try :help.
scala> var x = 10
x: Int = 10
scala> x
res0: Int = 10
scala> x = 11
x: Int = 11
scala> x
res1: Int = 11
scala>
However, Scala has a more interesting construct called val. Using the val operator makes your variables immutable, and this means you can't change the value once you've set it. If you try to change the value of the val variable in Scala, the compiler will give you an error. As a Scala developer, you should use the variable val as much as possible, because that's a good FP mindset, and it will make your programs better. In Scala, everything is an object; there are no primitives -- the var and val rules apply for everything it could but an Int or String or even a class.
We will see how to use val in Scala REPL as follows:
$ scala Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_77). Type in expressions for evaluation. Or try :help. scala> val x = 10 x: Int = 10 scala> x res0: Int = 10 scala> x = 11 <console>:12: error: reassignment to val x = 11 ^ scala> x res1: Int = 10 scala>