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
-
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 anHList
in linked list style.The
HList
cons cell, which represents one part of anHList
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 thisHList
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
-
sealed
trait
HList
extends AnyRef
The base trait for
HList
s.The base trait for
HList
s. Functions that takeHList
s will need a type parameter subtype ofHList
:def myHListFunction[T <: HList](list: HList) = { println(s"This HList has ${list.length} items!") }
-
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. -
final
class
HNil
extends HList
The last element of an
HList
.The last element of an
HList
. This is the starting point for anHList
, 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
Value Members
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final
def
!=(arg0: Any): Boolean
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final
def
##(): Int
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final
def
==(arg0: Any): Boolean
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val
HNil: HNil
The HNil singleton.
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final
def
asInstanceOf[T0]: T0
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def
clone(): AnyRef
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def
eq(arg0: AnyRef): Boolean
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equals(arg0: Any): Boolean
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finalize(): Unit
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isInstanceOf[T0]: Boolean
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ne(arg0: AnyRef): Boolean
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def
notify(): Unit
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def
notifyAll(): Unit
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def
synchronized[T0](arg0: ⇒ T0): T0
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def
toString(): String
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def
wait(): Unit
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def
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