How to correctly type-annotate this HList?

问题:

sealed abstract trait HList

case class :+:[H, T <: HList](head: H, tail: T) extends HList {
  def :+:[T](v: T) = new :+:(v, this)
}

case object HNil extends HList {
  def :+:[T](v: T) = new :+:(v, this)
}

object HListExpt {
  def main(args: Array[String]) {
    val me: String :+: Int :+: Symbol :+: HNil.type = "Rahul" :+: 20 :+: 'Male :+: HNil
    println(me.head, me.tail.head)
  }
}

On trying to compile the above code, I get the following compiler error:

error: type mismatch;
found   : :+:[java.lang.String,:+:[Int,:+:[Symbol,object HNil]]]
required: :+:[String,:+:[Int,:+:[Symbol,HNil.type]]]
val me: String :+: Int :+: Symbol :+: HNil.type = "Rahul" :+: 20 :+: 'Male :+: HNil

What am I doing wrong here? What would be the correct way to type-annotate the above HList?

PS: The code compiles fine when I remove the type annotation.

回答1:

The root problem here is that singleton types are never inferred. Here's a demonstration:

scala> case object A      
defined module A

scala> A                  
res6: A.type = A

scala> identity[A.type](A)
res7: A.type = A

scala> identity(A)        
res8: object A = A

Why is this? Quoth Odersky et. al. in Programming in Scala, §27.6:

Usually [singleton] types are too specific to be useful, which is why the compiler is reluctant to insert them automatically.

So, let's explicitly provide the type argument:

sealed abstract trait HList

case class :+:[H, T <: HList](head: H, tail: T) extends HList {
  def :+:[T](v: T) = new :+:(v, this)
}

case object HNil extends HList {
  def :+:[T](v: T) = new :+:[T, HNil.type](v, this)
}

val me: String :+: Int :+: Symbol :+: HNil.type = "Rahul" :+: 20 :+: 'Male :+: HNil
println(me.head, me.tail.head)

Bonus Link:

• Singleton Types are Mean and Spiteful

回答2:

I'm not sure why, but if HNil is defined as a class everything compiles:

class HNilClass extends HList {
  def :+:[T](v: T) = new :+:(v, this)
}

object HNil extends HNilClass