CODEWITHSUNDEEP
scalagenericscovariance
nix
nix

Reputation: 378

How can I remove this extra type parameter?

I currently have:

class X[+T: Numeric](val x: T)
abstract class M[N: Numeric, T <: X[N]] { // <- I'd like to remove N.
  def apply(x: Int): T
  final def row = (1 to 10).map(this(_))
}

I use it like this:

class Y(x: Double, val y: Double) extends X[Double](x)
class Z extends M[Double, Y] {           // <- So that this is simpler.
  def apply(x: Int) = new Y(0.0, 0.0)
}

It works like this:

object testapp {
  // row is properly polymorphic, allowing access to Y.y
  def main(args: Array[String]): Unit = (new Z).row.map(r => println(r.y))
}

I want Z to be simpler so that I can use M like:

class Z extends M[Y] {
  def apply(x: Int) = new Y(0.0, 0.0)
}

or, even better:

class Z extends M[Double] {           // i.e. Meaning apply can return
  def apply(x: Int) = new Y(0.0, 0.0) // any subclass of X[Double]
}

Here are my Gist iterations to reach this point.

Upvotes: 2

Views: 323

Answers (2)

som-snytt
som-snytt

Reputation: 39577

A third way in type params vs type members is to use both.

An advantage of a type member is that it doesn't pollute the signature of child classes. The type member can remain abstract if it is superfluous (even in a concrete class); and only the bottom class must define it if necessary.

  import scala.collection.immutable.IndexedSeq
  class X[+T: Numeric](val x: T)
  abstract class M[+A: Numeric] {
    type W <: X[A]
    def apply(x: Int): W
    final def row: IndexedSeq[W] = (1 to 10) map apply
    def sumx: A = {  // in terms of the underlying Numeric
      val n = implicitly[Numeric[A]]
      n fromInt (0 /: row)((s,w) => s + (n toInt w.x))
    }
  }

  class Y(x: Double, val y: Double) extends X[Double](x)
  class Z extends M[Double] {
    type W = Y
    def apply(x: Int) = new Y(0.0, 0.0)
  }

  def main(args: Array[String]): Unit = (new Z).row foreach (Console println _.y)

Upvotes: 2

Sergey Passichenko
Sergey Passichenko

Reputation: 6920

You didn't really need class M here:

class X[+T: Numeric](val x: T)
def row[W <: X[_]](c: => Int => W) = (1 to 10).map(c)

class Y(x: Double, val y: Double) extends X[Double](x)
def z = row(_ => new Y(0.0, 0.0))

def main(args: Array[String]): Unit = z.map(r => println(r.y))

If you want to keep M, you use same idea:

class X[+T: Numeric](val x: T)
abstract class M[W <: X[_]] {
    def apply(x: Int): W
    final def row = (1 to 10).map(this(_))
}

class Y(x: Double, val y: Double) extends X[Double](x)
class Z extends M[Y] {
  def apply(x: Int) = new Y(0.0, 0.0)
}

def main(args: Array[String]): Unit = (new Z).row.map(r => println(r.y))

Upvotes: 1

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