问题
I was trying to define a type that accept an existential higher kinded type in Scala.
Unfortunately Scalac does not allow it.
Welcome to Scala version 2.11.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_45).
Type in expressions to have them evaluated.
Type :help for more information.
scala> :paste
// Entering paste mode (ctrl-D to finish)
trait H[F[_, _]]
trait T[A, B]
val h:H[T] = null
val e:H[F] forSome { type F[A, B] } = h
// Exiting paste mode, now interpreting.
<console>:13: error: type mismatch;
found : H[T]
required: H[_[A, B] <: Any]
Note: T <: Any, but trait H is invariant in type F.
You may wish to define F as +F instead. (SLS 4.5)
val e:H[F] forSome { type F[A, B] } = h
^
How could I workaround it?
How could I define a type that matches any H whatever the type parameter is?
Update: I even tried to let Scalac infer the existential type, still not lucky.
Welcome to Scala version 2.11.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_45).
Type in expressions to have them evaluated.
Type :help for more information.
scala> :paste
// Entering paste mode (ctrl-D to finish)
trait H[F[_, _]]
trait T[A, B]
val h:H[T] = null
trait T2[A, B]
val h2:H[T2] = null
Seq(h, h2)
// Exiting paste mode, now interpreting.
<console>:17: error: type mismatch;
found : H[T]
required: H[_ >: T2 with T <: Object]
Seq(h, h2)
^
<console>:17: error: type mismatch;
found : H[T2]
required: H[_ >: T2 with T <: Object]
Seq(h, h2)
^
回答1:
Probably sschaef is right, this is not possible. I don't know if the following helps you in any way:
type F[A,B] = X[A,B] forSome { type X[A,B] } // alias
trait H[F[_, _]]
trait T[A, B]
val h: H[T] = null
val e: H[F] = h.asInstanceOf[H[F]]
回答2:
I found that existential higher-kinded types can be written in type patterns (and inferred from there):
def toExistential[F[_, _]](h: H[F]) = h.asInstanceOf[Any] match {
case x: H[_] => x
}
> val h = new H[T] {}
h: H[T] = $anon$1@4d0c3eac
> val f = toExistential(h)
f: H[_] = $anon$1@4d0c3eac
The cast to Any seems to be necessary, but at least it can be encapsulated in a function.
回答3:
Instead of a type parameter you could use a type member:
trait H {
type F[_, _]
}
来源：https://stackoverflow.com/questions/32182694/how-to-define-an-existential-higher-kinded-type-in-scala