object HLists

Basic support for heterogeneous lists, aka HLists.

An HList can be constructed like so:

import net.liftweb.common.HLists._

trait Base
case class Type1(value: String) extends Base
case class Type2(otherValue: String) extends Base

val myHList = Type1("Value") :+: Type2("Other Value") :+: HNil
myHList match {
  case firstThing :+: secondThing :+: HNil =>
    println(firstThing.value)
    println(secondThing.otherValue)
}

Above, we see that the HList preserved the value of the types of its members, otherwise we wouldn't have been able to fetch value and otherValue, respectively.

Trying the same thing with a list won't work:

val myList = Type1("Value") :: Type2("Other Value") :: Nil
myList match {
  case firstThing :: secondThing :: Nil =>
    // error: value value is not a member of Product with Serializable with Base
    println(firstThing.value)
}

This is because value is not defined in Base. The inferred type of the List has to be a common ancestor class or trait of Type1 and Type2, and no such type has a value method.

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Type Members

  1. final case class :+: [+H, +T <: HList](head: H, tail: T) extends HList with Product with Serializable

    The HList cons cell, which represents one part of an HList in linked list style.

    The HList cons cell, which represents one part of an HList in linked list style.

    Carries the information about the type of this element, plus the HList type of the rest of the list.

    You can use :+: to make this HList longer:

    scala> val first = Type1("Value") :+: HNil
first: net.liftweb.common.HLists.HCons[Type1,net.liftweb.common.HLists.HNil] = Type1(Value) :+: HNil
scala> Type2("Other Value") :+: first
res0: net.liftweb.common.HLists.HCons[Type2,
        net.liftweb.common.HLists.HCons[Type1,
          net.liftweb.common.HLists.HNil]] =
      Type2(Other Value) :+: Type1(Value) :+: HNil
  2. sealed trait HList extends AnyRef

    The base trait for HLists.

    The base trait for HLists. Functions that take HLists will need a type parameter subtype of HList:

    def myHListFunction[T <: HList](list: HList) = {
  println(s"This HList has ${list.length} items!")
}
  3. implicit final class HListMethods [ListSoFar <: HList] extends AnyRef

    Provides the methods that can be used on an HList.

    Provides the methods that can be used on an HList. These are set apart here due to certain issues we can experience otherwise with the type variance on the :+: class.

  4. final class HNil extends HList

    The last element of an HList.

    The last element of an HList. This is the starting point for an HList, and you can use :+: to start one based on it:

    scala> Type1("Value") :+: HNil
res0: net.liftweb.common.HLists.HCons[Type1,net.liftweb.common.HLists.HNil] = Type1(Value) :+: HNil

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  1. final def !=(arg0: Any): Boolean
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  2. final def ##(): Int
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  3. final def ==(arg0: Any): Boolean
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  4. val HNil: HNil

    The HNil singleton.

  5. final def asInstanceOf[T0]: T0
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