Custom implementation of HList and print operation.

customhlist.scala

object foo extends App {                                                                                           
                                                                                                                   
  trait HList                                                                                                      
  trait HNil extends HList                                                                                         
  case object HNil extends HNil {                                                                                  
    def ::[H](h: H): H :: HNil = foo.::(h, HNil)                                                                   
  }                                                                                                                
  case class ::[H, T <: HList](h: H, t: T) extends HList { self =>                                                 
    def ::[H](h: H) = foo.::(h, self)                                                                              
  }                                                                                                                
                                                                                                                   
                                                                                                                   
  trait Print[L <: HList] {                                                                                        
    def apply(l: L): String                                                                                        
  }                                                                                                                
                                                                                                                   
  object Print {                                                                                                   
                                                                                                                   
    def apply[L <: HList](implicit p: Print[L]) = p                                                                
                                                                                                                   
    implicit val pnil: Print[HNil] = new Print[HNil] {                                                             
      def apply(l: HNil) = "HNil"                                                                                  
    }                                                                                                              
    implicit def pl[H, T <: HList](implicit pt: Print[T]): Print[H :: T] = new Print[H :: T] {                     
      def apply(l: H :: T) = s"(${l.h}:${l.h.getClass.getSimpleName}) :: " + pt(l.t)                               
    }                                                                                                              
  }                                                                                                                
                                                                                                                   
  implicit class HListOps[L <: HList](l: L) {                                                                      
    def print(implicit p: Print[L]) = p(l)                                                                         
  }                                                                                                                
                                                                                                                   
  /*                                                                                                               
    scala> import foo._                                                                                            
    import foo._                                                                                                   
                                                                                                                   
    scala> 1 :: 2 :: false :: HNil                                                                                 
    res0: foo.::[Int,foo.::[Int,foo.::[Boolean,foo.HNil]]] = ::(1,::(2,::(false,HNil)))                            
                                                                                                                 
    scala> res0.print                                                                                              
    res1: String = (1:Integer) :: (2:Integer) :: (false:Boolean) :: HNil                                           
  */                                                                                                               
}