Why it is not possible to override mutable variable in scala?

Question

Why it is not possible to override mutable variable in scala ?

class Abs(var name: String){
}

class AbsImpl(override var name: String) extends Abs(name){
}

Above code gives following compile time error :-

variable name cannot override a mutable variable

If name is declared val, then above code works fine.

Answer 1

The short answer: you need to pass -Yoverride-vars to the Scala compiler.

According to the spec, a var is both a getter and a setter, and normal overriding rules apply for these methods. However, this proved to have some unwanted consequences w.r.t. to the final keyword and inlining. The code in the compiler mentions some spec clarifications would be needed:

// TODO: this is not covered by the spec. We need to resolve this either by changing the spec or removing the test here.
if (!settings.overrideVars)
  overrideError("cannot override a mutable variable")

A related ticket: SI-3770

Answer 2

If you could override a var with a var, then the overriding member could have a narrower type. (That's how overriding is defined.)

You could then assign a value of a wider type and then read it expecting the narrower type, and fail.

Illustration of the setter involved:

scala> class A ; class B extends A
defined class A
defined class B

scala> abstract class C { var x: A } ; class D extends C { var x: B = _ }
<console>:13: error: class D needs to be abstract, since variable x in class C of type A is not defined
(Note that an abstract var requires a setter in addition to the getter)
       abstract class C { var x: A } ; class D extends C { var x: B = _ }
                                             ^

scala> abstract class C { var x: A }
defined class C

scala> class D extends C { var x: B = _ ; def x_=(a: A) = ??? }
defined class D