[scala] Covariance in unsound
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[scala] Covariance in unsound
by nikov
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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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[scala] Re: Covariance in unsound
by Adriaan Moors-2
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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, |
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[scala] Re: Covariance in unsound
by nikov
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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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[scala] Re: Covariance in unsound
by Adriaan Moors-2
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I was referring to the type that you refine with. { type X = T } is a structural type, and because of that, the overriding checks are different.
adriaan
On Sat, Jun 20, 2009 at 11:23 AM, Vladimir Reshetnikov <v.reshetnikov@...> wrote: Hi Adriaan, |
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Re: [scala] Re: Covariance in unsound
by Matt Shannon
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Hi Vladimir,
I'm not suggesting it is in any way satisfactory, but a lot of your recent unsoundness examples can be fixed just by not using anonymous classes. For example, if you instead write: class B { type X var x : X = _ } class C[+T] { class D extends B { type X = T } val s = new D } then the compiler complains appropriately. Some of your other examples similarly get the correct result if you replace anonymous classes by explicitly named ones. Obviously this is not a fix. My suggestion is only that if you want to learn more about the intricacies of the scala type system, and don't want to wait until the bugs you highlighted are fixed, then you could try using explicitly named classes in your investigations. Matt Adriaan Moors wrote: > I was referring to the type that you refine /with./ { type X = T } is a > structural type, and because of that, the overriding checks are different. > > adriaan > > On Sat, Jun 20, 2009 at 11:23 AM, Vladimir Reshetnikov > <v.reshetnikov@... <mailto:v.reshetnikov@...>> wrote: > > 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@... > <mailto: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@... <mailto: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 > >> > > > > > Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm > > |
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[scala] Re: [scala-user] Covariance in unsound
by Martin Odersky
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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) > > tested for variance. I just fixed that, and will check it in on Monday. Thanks for discoverig this! Cheers -- Martin |
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