private[this] vs private

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There is a case where private[this] is required to make code compile. This has to do with an interaction of variance notation and mutable variables. Consider the following (useless) class:

class Holder[+T] (initialValue: Option[T]) {
    // without [this] it will not compile
    private[this] var value = initialValue

    def getValue = value
    def makeEmpty { value = None }
}

So this class is designed to hold an optional value, return it as an option and enable the user to call makeEmpty to clear the value (hence the var). As stated, this is useless except to demonstrate the point.

If you try compiling this code with private instead of private[this] it will fail with the following error message:

error: covariant type T occurs in contravariant position in type Option[T] of value value_=
class Holder[+T] (initialValue: Option[T]) {

This error occurs because value is a mutable variable on the covariant type T (+T) which is normally a problem unless marked as private to the instance with private[this]. The compiler has special handling in its variance checking to handle this special case.

So it’s esoteric but there is a case where private[this] is required over private.

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