[scala] Re: Covariance in unsound

Hi Adriaan,

In my example the refinement is not structural - the overridden
virtual type exists in the base class.

Vladimir

On 6/20/09, Adriaan Moors <adriaan.moors@...> wrote:
> Hi Vladimir,
> Could you add this example to
> https://lampsvn.epfl.ch/trac/scala/ticket/963, they
> seem to be related. Override checks are too lax in structural refinements
> --
> your example doesn't type check when you simply write:
>
> scala> class C[+T] {
>      | type X = T
>      | }
> <console>:5: error: covariant type T occurs in invariant position in type T
> of type X
>        type X = T
>             ^
>
>
> thanks!
> adriaan
>
> On Sat, Jun 20, 2009 at 8:23 AM, Vladimir Reshetnikov <
> v.reshetnikov@...> wrote:
>
>> Hi,
>>
>> It seems that the current implementation of covariance is unsound.
>> Consider this code:
>>
>> ///////////////////////////////////////////////
>> class B {
>>  type X
>>  var x : X = _
>> }
>> class C[+T] {
>>  val s = new B { type X = T }
>> }
>> ///////////////////////////////////////////////
>>
>> It compiles without errors in Scala 2.8.0.r18050-b20090618020144, but
>> then we can write:
>>
>> scala> val c = new C[Int]
>> c: C[Int] = C@1046270
>>
>> scala> val c1 = c : C[Any]
>> c1: C[Any] = C@1046270
>>
>> scala> c1.s.x = ""
>>
>> scala> c.s.x
>> java.lang.ClassCastException: java.lang.String cannot be cast to
>> java.lang.Integer
>>        at scala.runtime.BoxesRunTime.unboxToInt(Unknown Source)
>>
>>
>> I would like to read some papers on theoretical foundations of
>> covariance in Scala. What would you recommend?
>>
>> Thanks,
>> Vladimir
>>
>>
>> Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm
>>
>


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