How to replace static variables with scala available means in a base class

Question

What I need:

trait Base {
  import scala.math.{Pi=>mPi, cos=>msoc, sin=>msin}
  static val Pi : Float = mPi.toFloat
  static def cos(phi : Float) : Float = mcos(phi).toFloat
  static def sin(phi : Float) : Float = msin(phi).toFloat
  // ...
  // some useful functions
}
// children use float defined pi, cos and sin without additional boilerplate
class Child1 extends Base
// ...
class Child2 extends Base

That obviously would not work. What I tried

trait Base {
  import Base._
  // some useful functions
}
object Base {
  // utility functions
  import scala.math.{Pi=>mPi, cos=>mcos, sin=>msin}
  val Pi : Float = mPi.toFloat
  def cos(phi : Float) : Float = mcos(phi).toFloat
  def sin(phi : Float) : Float = msin(phi).toFloat
}
class Child1 extends Base {
  // use sin and cos inside
}

And the compiler gives errors in Child1 that it does nothing about functions sin and cos.

How can I define a static members in a base class that would be inherited by its descendants?

What about adding the import statement at the beginning of the file? — kiritsuku
– kiritsuku, Commented Jan 12, 2013 at 18:24

Randall Schulz · Accepted Answer · 2013-01-12 17:32:22Z

1

Why are you thinking in terms of static? All you need to do is create a method with no argument list that returns the Float forms of your constants.

For better or worse, as you've discovered, imports in a class or trait scope a not inherited.

package rrs.scribble

trait   FloatMath
{
  import scala.math.{Pi => mPi, cos => mcos, sin => msin}

  def Pi: Float = mPi.toFloat
  def cos(phi: Float): Float = mcos(phi).toFloat
  def sin(phi: Float): Float = msin(phi).toFloat
}

object  FM1
extends FloatMath
{
  val twoPi = 2.0f * Pi
}

In the REPL:

scala> import rrs.scribble.FM1._
import rrs.scribble.FM1._

scala> twoPi
res0: Float = 6.2831855

scala> cos(twoPi)
res1: Float = 1.0

As you see, the static types of Pi, twoPi, cos, etc. are all Float, not Double.

answered Jan 12, 2013 at 17:32

Randall Schulz

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2 Comments

ayvango Over a year ago

I may not replace val with a function def. As I've experimented with scala inheritance system, a I may use val instead of def, but vise versa would be denied by type system. So val and def are essentially different entities, and probably def is more expensive than val

Randall Schulz Over a year ago

Incorrect. If you use val to hold a constant, every instance will have a (redundant) copy while with a def it will be part of the bytecode and shared by all instances. Furthermore, access to val still goes through an accessor method. Either way, the JIT native code compilation is likely to make them identical at run time.

Kazuhiro Sera · Accepted Answer · 2013-01-12 15:45:20Z

1

How about choosing either mixing in a trait or importing members from an object?

And I think the latter is better.

trait MathUtils { 
  def cos(phi : Float) : Float = ...
}
class Child1 extends MathUtils { cos(1.0F) }

or

object MathUtils { 
  def cos(phi : Float) : Float = ...
}
class Child1 { 
  import MathUtils._
  cos(1.0F)
}

answered Jan 12, 2013 at 15:45

Kazuhiro Sera

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1 Comment

ayvango Over a year ago

I'm using method #2 for now, but it bothers me, since children are really small and import statement stand out too much

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