具有更高种类类型的类的自类型注释 [英] Self-type annotation for class with higher kinded type

问题描述

给定:

abstract class Databases[F[_]]

我怎样才能使这个特性起作用:

How can I make this trait work:

// Marker trait signalling the database plugin supports StaticRoles
trait StaticRoles { this: Databases[_] => }

我想确保 StaticRoles 只混合在扩展 Databases 的类中，但是我不关心类型参数 的具体值>F.

I want to ensure StaticRoles is only mixed-in in classes that also extend Databases, however I don't care about the concrete value of the type parameter F.

代码返回:

error: _$1 takes no type parameters, expected: one

这是公平的，但它返回相同的错误:

Which is fair, however it returns the same error for:

trait StaticRoles { this: Databases[_[_]] => }

我也试过:

trait StaticRoles { this: Databases[({type T[X[_]]})#T] => }

哪个给出了错误:

error: kinds of the type arguments (AnyRef{type T[X[_]]}#T) do not conform to the expected kinds of the type parameters (type F) in class Databases.
       AnyRef{type T[X[_]]}#T's type parameters do not match type F's expected parameters:
       type X has one type parameter, but type _ has none

推荐答案

正确的是

trait StaticRoles { this: (Databases[F] forSome { type F[_] }) => }

是否有简写类型变量 'm forSome { type m[O] <: UpperBound[O] }` 在 Scala 中?

并非所有存在类型都可以用下划线表示，或者(在这种情况下)允许用下划线表示.

Not every existential type is expressible with underscores or (in this case) allowed to be expressed with underscores.

还有

trait StaticRoles { this: Databases[F forSome { type F[_] }] => }

与前者不同但相同

trait StaticRoles { this: Databases[Any] => }

自从

implicitly[(Databases[F forSome { type F[_] }]) =:= Databases[Any]]

(Any 实际上是 poly-kinded).

(Any is actually poly-kinded).

Databases[Any] 是 Databases[F] forSome { type F[_] }

implicitly[Databases[Any] <:< (Databases[F] forSome { type F[_] })]

使用类型投影 (#) 正确的是

With type projections (#) correct is

trait StaticRoles { this: Databases[({ type F[_] })#F] => }

Databases[({ type F[_] })#F] 也是 Databases[F] forSome { type F[_] } 的子类型(不可比使用 Databases[Any] 表示不变的 Databases).

Databases[({ type F[_] })#F] is also a subtype of Databases[F] forSome { type F[_] } (uncomparable with Databases[Any] for invariant Databases).

在这三种类型中Databases[F] forSome { type F[_] }、Databases[Any]和Databases[({ type F[_] })#F] 只有第一个适用

Among these three types Databases[F] forSome { type F[_] }, Databases[Any], and Databases[({ type F[_] })#F] only the first one works with

trait IO[_]

class Abc extends Databases[IO] with StaticRoles // compiles

//class Abc1 extends StaticRoles // doesn't compile

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