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[scala] Re: Covariance in unsound

by Adriaan Moors-2 :: Rate this Message:

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

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