Oops, missed the cast to an A. Indeed that's a problem
trait A {
def f[T[_]](x : T[Int]) : T[Any]
}
class B extends A {
def f[T[+_]](x : T[Int]) : T[Any] = x
}
class P[Y](var y : Y)
val p = new P(1)
val palias = (new B():A).f[P](p)
palias.y = "hello"
scala> p.y
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
On Tue, Jun 16, 2009 at 6:51 AM, James Iry
<jamesiry@...> wrote:
Which version of Scala are you running? In both 2.7.5 and 2.8.0.something I get
<console>:7: error: kinds of the type arguments (P) do not conform to the expected kinds of the type parameters (type T).
P's type parameters do not match type T's expected parameters: type Y is invariant, but type _ is declared covariant
val palias = new B().f[P](new P[Int](1))On Tue, Jun 16, 2009 at 6:27 AM, Vladimir Reshetnikov
<v.reshetnikov@...> wrote:
Hi,
Consider the following code:
//////////////////////////////////////////////////
trait A {
def f[T[_]](x : T[Int]) : T[Any]
}
class B extends A {
def f[T[+_]](x : T[Int]) : T[Any] = x
}
class P[Y](var y : Y)
//////////////////////////////////////////////////
It accepted by the Scala 2.7.5 compiler without any errors. But is
seems unsound, because then we can write (new B:A).f[P](new
P[Int](1)), and nonvariant P[Int] is coerced to P[Any].
Thanks,
Vladimir
