Confused with Variance
Given the following:
trait Fruit
class Apple extends Fruit
class Orange extends Fruit
case class Crate[T](value:T)
def p(c:Crate[Fruit]) { }
val cra = Crate(new Apple)
val cro = Crate(new Orange)
since Crate 开发者_高级运维is invariant, I can't do the following (as expected):
scala> val fruit:Crate[Fruit] = cra
<console>:10: error: type mismatch;
found : Crate[Apple]
required: Crate[Fruit]
val fruit:Crate[Fruit] = cra
^
scala> val fruit:Crate[Fruit] = cro
<console>:10: error: type mismatch;
found : Crate[Orange]
required: Crate[Fruit]
val fruit:Crate[Fruit] = cro
scala> p(cra)
<console>:12: error: type mismatch;
found : Crate[Apple]
required: Crate[Fruit]
p(cra)
^
scala> p(cro)
<console>:12: error: type mismatch;
found : Crate[Orange]
required: Crate[Fruit]
p(cro)
But why can I call method p with these when Crate is not covariant? :
scala> p(Crate(new Apple))
Crate(line2$object$$iw$$iw$Apple@35427e6e)
scala> p(Crate(new Orange))
Crate(line3$object$$iw$$iw$Orange@33dfeb30)
Have I missed some basic principles of variance?
In the latter cases, the compiler assumes that you want this to work and actually says
p(Crate( (new Apple): Fruit ))
which is perfectly okay. It's the same as if you manually did
val f: Fruit = new Apple // totally fine
p(Crate(f)) // Also totally fine
This is just a small part of the immense wizardry that the compiler applies to try to figure out what you mean with your types without making you type it all out.
精彩评论