CODEWITHSUNDEEP
scalahigher-kinded-types
Patrik
Patrik

Reputation: 355

Scala higher-kinded types general factory

I'v been playing around with scala again testing some language type check features. I'm trying to implement a vector library intened for use with graphics and I want to use the scala typechecker as much as possible to get early compile time warnings when using it. What I'v got so far.

trait VecT

abstract sealed class Vec[T,V[T] <: VecT](elems: T*)(implicit num: VecIntegral[T]) extends VecT {
  import num._

  def +(v: V[T]): V[T] = ???
  def -(v: V[T]): V[T] = ???
  def cross(v: V[T]): V[T] = ???
  def dot(v: V[T]): T = ???
  def unary_-(): V[T] = ???
  def *(scalar: T): V[T] = ???
  def abs: T = ???
  def *(v: V[T]): V[T] = cross(v)
  def apply(n: Int): T = ???
}

class Vec2[T](x: T, y: T)(implicit num: VecIntegral[T]) extends Vec[T, Vec2](x,y)
class Vec3[T](x: T, y: T, z: T)(implicit num: VecIntegral[T]) extends Vec[T, Vec3](x,y,z)

this allows me to do compile time checking for operations like even if the + operator is implemented in the abstract class

(new Vec3[Int](1,2,3)) + (new Vec3[Int](1,2,3))
(new Vec3[Int](1,2,3)) + (new Vec2[Int](1,2)) // Failes compilation

which is what i want.

So far so good. Now i would like to have a map function implemented in the abstract class something like

def map[A](f: T => A): V[A] = Vec(elems.map(f):_*)

So my attempt of realizing this would be to create a general Vec factory 

object Vec {
  def apply[T, V[T] <: VecT](elemes: T*)(implicit num: VecIntegral[T]): V[T] = elems match {
    case Seq(x,y)   => new V[T](x,y)
    case Seq(x,y,z) => new V[T](x,y,z)
}

but that wont work, expression of type V does not comform to exptected type V[T]

So my question. Is there any "good" general way of implementing a factory like this?

Upvotes: 0

Views: 63

Answers (1)

Alexey Romanov
Alexey Romanov

Reputation: 170713

For why new V[T](...) doesn't and shouldn't work, see https://stackoverflow.com/a/39286308/9204. One solution would be

trait VecFactory[V[_]] { def newVec[T](elems: Seq[T]): V[T] }

abstract sealed class Vec[T,V[T] <: VecT](elems: T*)(implicit num: VecIntegral[T], factory: VecFactory[V]) {
  def map[A](f: T => A): V[A] = factory.newVec(elems.map(f))
}
class Vec2[T](x: T, y: T)(implicit num: VecIntegral[T]) extends Vec[T, Vec2](x,y)
object Vec2 {
  implicit val factory: VecFactory[Vec2] = new VecFactory[Vec2] { 
    def newVec[T](elems: Seq[T]) = new Vec2(elems(0), elems(1))
  }
}

class Vec3[T](x: T, y: T, z: T)(implicit num: VecIntegral[T]) extends Vec[T, Vec3](x,y,z)
object Vec3 {
  implicit val factory: VecFactory[Vec3] = ...
}

Note that this still isn't perfectly safe: the factories need to be called with sequences of specific length.

Upvotes: 1

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