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/*
 * 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 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)
  }

}

// Some useful type system stuff from Miles Sabin
/**
 * Encoding for "A is not a subtype of B"
 */
sealed trait ExcludeThisType[A, B]

/**
 * The companion object to <:!<. This allows one of specify
 * that a type is not a subtype of another type
 */
object ExcludeThisType {
  def unexpected: Nothing = sys.error("Unexpected invocation")

  // Uses ambiguity to rule out the cases we're trying to exclude
  implicit def nsub[A, B]: A ExcludeThisType B = null

  implicit def `This type was excluded because it was explicitly excluded`[A, B >: A]: A ExcludeThisType B = unexpected

  implicit def `Ignore me, I only exist to cause the compiler to fail`[A, B >: A]: A ExcludeThisType B = unexpected

  // Type alias for context bound
  type exclude[T] = {
    type other[U] = U ExcludeThisType T
  }
}