作为类型成员的单例类型

Question

我正在尝试将类型成员定义为单例类型。这是我的尝试：

import shapeless.syntax.singleton._

trait Test{
    type Blocked <: Boolean
}

def f(t: Test{ type Blocked = false.narrow }) = ??? // does not compile

def f(t: Test{ type Blocked = false }) = ??? // argument is parsed as t: Test{ type Blocked = Boolean }

有没有办法将类型成员设置为单例类型（Boolean(false) 在我的例子中）？

Answer 1

def f(t: Test{ type Blocked = false }) = ??? // argument is parsed as t: Test{ type Blocked = Boolean }

Blocked 变成文字单例类型 false；请注意 f(true) 是如何被拒绝的

scala> trait Test{
     |     type Blocked <: Boolean
     | }
trait Test

scala> val v = new Test { type Blocked = false }
val v: Test{type Blocked = false} = $anon@34aa8b61

scala> def f(x: v.Blocked) = x
def f(x: v.Blocked): v.Blocked

scala> f(false)
val res0: v.Blocked = false

scala> f(true)
         ^
       error: type mismatch;
        found   : Boolean(true)
        required: v.Blocked
           (which expands to)  false

作为类型成员的单例类型

Singleton type as a type member

scala

shapeless

type-safety

refined