我想做一些类型级别的编程。Scala 类型的宏已经死了，看起来 shapeless 无法做我想做的事。如果我能让以下代码工作，我所有的问题都将得到解决（此代码是从http://apocalisp.wordpress.com/2010/06/08/type-level-programming-in-scala/和http 借来的： //michid.wordpress.com/2010/06/18/type-level-programming-equality/ ) 它显示了具有“类型删除”运算符的 HList 的实现。该错误来自“EQ”结构，该结构测试两种类型是否等效以便删除。在我的情况下，它似乎总是返回 false。我认为这是因为我使用了“别名”（？），但我不知道如何克服这个困难。
class Root {
def typed[T](t: => T) {}
}
trait BooleanCompute extends Root {
trait BOOL {
type OUT <: BOOL
type SELECT[TrueAction <: Action, FalseAction <: Action, Action] <: Action
}
case class TRUE() extends BOOL {
type OUT = TRUE
type SELECT[TrueAction <: Action, FalseAction <: Action, Action] = TrueAction
}
case class FALSE() extends BOOL {
type OUT = FALSE
type SELECT[TrueAction <: Action, FalseAction <: Action, Action] = FalseAction
}
}
object BooleanCompute extends BooleanCompute
import BooleanCompute._
case class Equality[A]() {
def check(x: A)(implicit t: TRUE) = t
def check[B](x: B)(implicit f: FALSE) = f
}
object Equality {
def witness[T] = null.asInstanceOf[T]
implicit val t: TRUE = null
implicit val f: FALSE = null
}
trait EQ[A, B] extends {
import Equality._
type OUT = out.type
val out = Equality[A]() check witness[B]
val test1 = Equality[List[Boolean]] check witness[List[Boolean]]
implicitly[test1.OUT =:= TRUE]
// Does not compile since tt is True
//implicitly[test1.t =:= FALSE]
val test2 = Equality[Nothing] check witness[AnyRef]
// Does not compile since ft is False
// implicitly[test2.t =:= TRUE]
implicitly[test2.OUT =:= FALSE]
}
object HList extends Root {
type :+:[H, T <: HList] = HCons[H, T]
val :+: = HCons
sealed trait HList {
self: HList =>
type SELECT[ConsAction <: Action, NilAction <: Action, Action] <: Action
type Remove[A] <: HList
}
final case class HCons[Head, Tail <: HList](head: Head, tail: Tail) extends HList {
self: HList =>
type SELECT[ConsAction <: Action, NilAction <: Action, Action] = ConsAction
type Remove[A] = SELECT[
EQ[Head, A]#OUT#SELECT[Tail#Remove[A], Head :+: Tail#Remove[A], HList],
HNil,
HList]
def :+:[T](v: T) = HCons(v, this)
}
sealed case class HNil() extends HList {
type SELECT[ConsAction <: Action, NilAction <: Action, Action] = NilAction
type Remove[A] = HNil
def :+:[T](v: T) = HCons(v, this)
}
}
object yo extends App {
import HList._
case class AA()
case class BB()
case class CC()
case class DD()
type YO = AA :+: BB :+:CC :+: HNil
type YoRemoved = AA :+: CC :+: HNil
val a = AA() :+: CC() :+: HNil()
typed[YoRemoved](a)
typed[YO#Remove[BB]](a)
}