Heterogenous Lists

Heterogenous lists

shapeless provides a comprehensive Scala HList which has many features not shared by other HList implementations.

It has a map operation, applying a polymorphic function value across its elements. This means that it subsumes both typical HList's and also KList's (HList's whose elements share a common outer type constructor).

import poly._

object choose extends (Set ~> Option) {
  def apply[T](s: Set[T]) = s.headOption

val sets = Set(1) :: Set("foo") :: HNil

val opts = sets map choose

opts should be(res0 :: res1 :: HNil)

It also has a flatMap Operation

import poly.identity

val l = (23 :: "foo" :: HNil) :: HNil :: (true :: HNil) :: HNil

l flatMap identity should be(res0 :: res1 :: res2 :: HNil)

It has a set of fully polymorphic fold operations which take a polymorphic binary function value. The fold is sensitive to the static types of all of the elements of the HList. Given the earlier definition of size,

object addSize extends Poly2 {
  implicit def default[T](implicit st: shapelessex.size.Case.Aux[T, Int]) =
    at[Int, T] { (acc, t) => acc + size(t) }
val l = 23 :: "foo" :: (13, "wibble") :: HNil

l.foldLeft(0)(addSize) should be(res0)

It also has a zipper for traversal and persistent update,

import syntax.zipper._

val l = 1 :: "foo" :: 3.0 :: HNil
l.toZipper.right.put(("wibble", 45)).reify should be(res0 :: res1 :: res2 :: HNil)

l.toZipper.right.delete.reify should be(res3 :: res4 :: HNil)

It is covariant,

object CovariantHelper {

  trait Fruit
  case class Apple() extends Fruit
  case class Pear() extends Fruit

  type FFFF = Fruit :: Fruit :: Fruit :: Fruit :: HNil
  type APAP = Apple :: Pear :: Apple :: Pear :: HNil

  val a: Apple = Apple()
  val p: Pear = Pear()

  val apap: APAP = a :: p :: a :: p :: HNil

import scala.reflect.runtime.universe._

implicitly[TypeTag[APAP]].tpe.typeConstructor <:< typeOf[FFFF] should be(res0)

And it has a unify operation which converts it to an HList of elements of the least upper bound of the original types,

apap.isInstanceOf[FFFF] should be(res0)
apap.unify.isInstanceOf[FFFF] should be(res1)

It supports conversion to an ordinary Scala List of elements of the least upper bound of the original types,

apap.toList should be(res0)

And it has a Typeable type class instance (see below), allowing, eg. vanilla List[Any]'s or HList's with elements of type Any to be safely cast to precisely typed HList's. These last three features make this HList dramatically more practically useful than HList's are typically thought to be: normally the full type information required to work with them is too fragile to cross subtyping or I/O boundaries. This implementation supports the discarding of precise information where necessary. (eg. to serialize a precisely typed record after construction), and its later reconstruction. (eg. a weakly typed deserialized record with a known schema can have its precise typing reestablished).

import syntax.typeable._

val ffff: FFFF = apap.unify
val precise: Option[APAP] = ffff.cast[APAP]

precise should be(res0)