1

as expected reassignment is giving error like below in REPL

scala> val a=1
a: Int = 1

scala> a=2
<console>:12: error: reassignment to val
       a=2
        ^

But the below reassignment is not giving error in REPL when a=2 preceded with val.

scala> val a=1
a: Int = 1

scala> val a=2
a: Int = 2

When I execute the below code in Intellij its giving error.

object Test {
  def main(args: Array[String]) {
    val x = 1
    val x = 2
  }
}

Why val a=1 and val a=2 are not giving any error in REPL(error if it is only a=2) but error in Intellij.

Tomer Shetah
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dsk
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  • The **REPL** allows you to shadows old definitions with new values, this is useful for experimenting but could lead to obscure code, that is why this only works on a **REPL**. - BTW, note that you can emulate that behaviour in code too like this `{ val x = 1 { val x = 2 } }` note the inner block, each block can shadow previously defined variables, but once you exit the block the old variable exists again. – Luis Miguel Mejía Suárez Nov 03 '20 at 17:15

2 Answers2

4

From Scala docs REPL overview:

  • every line of input is compiled separately.
  • dependencies on previous lines are included by automatically generated imports.

Combining these two facts, we can understand that they are not in the same namespace, unlike the example you provided which 2 variables called x are in the same class.

Tomer Shetah
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2

The REPL is intended for rapid friction-less experimentation. It would be very annoying if you had to restart from scratch just because you accidentally mistyped val a = 32 when you meant val a = 23.

Therefore, the REPL is designed in such a way that it gives the appearance of breaking the rules of Scala, although it actually doesn't. The code that gets actually compiled corresponding to the code you entered looks a little bit like this:

object line$1 {
  val a=1
}

object line$2 {
  import line$1._
  val a=2
}
Jörg W Mittag
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