Lift Framework example source code file (HList.scala)

This example Lift Framework source code file (HList.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Lift Framework tags/keywords

h, hcons, hcons, head, hlist, hlist, hlists, hnil, hnil, int, t, t, tail, this

The Lift Framework HList.scala source code

/*
 * Copyright 2010-2011 WorldWide Conferencing, LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package net.liftweb 
package common 

/**
 * Support for heterogenious lists, aka <a href="http://apocalisp.wordpress.com/2010/07/06/type-level-programming-in-scala-part-6a-heterogeneous-list%C2%A0basics/">HLists
 * 
 */
object HLists {
  /**
   * The trait that defines HLists
   */
  sealed trait HList {
    type Head
    type Tail <: HList

    /**
     * The length of the HList
     */
    def length: Int
  }

  /**
   * The last element of an HList
   */
  final class HNil extends HList {
    type Head = Nothing
    type Tail = HNil
    
    /**
     * Create a new HList based on this node
     */
    def :+:[T](v : T) = HCons(v, this)

    override def toString = "HNil"

    /**
     * The length of the HList
     */
    def length = 0
  }

  /**
   * The HNil singleton
   */
  val HNil = new HNil()

  /**
   * The HList cons cell
   */
  final case class HCons[H, T <: HList](head : H, tail : T) extends HList {
    type This = HCons[H, T]
    type Head = H
    type Tail = T
    
    /**
     * Create a new HList based on this node
     */
    def :+:[T](v : T) = HCons(v, this)

    override def toString = head + " :+: " + tail

    /**
     * The length of the HList
     */
    def length = 1 + tail.length
  }

  type :+:[H, T <: HList] = HCons[H, T]
  object :+: {
    def unapply[H, T <: HList](in: HCons[H, T]): Option[(H, T)] = Some(in.head, in.tail)
    }
  
}