Re: Sealed trait hierarchies and downcasting

On Mon, 2015-03-02 at 07:32 -0800, rklaehn wrote: 

> I want Foo to be a pure trait / interface, and not have any
> implementation there. Tree will contain implementation and private
> helper methods. E.g.
> 
> sealed trait Foo { def size: Int }
> 
> private sealed class Tree extends Foo {
>   final def size:Int = this match { case Leaf(_) => 1; case Branch(l,
> r) => l.size + r.size }
> }


For this purpose, I suggest replacing class Tree with


private implicit final class Tree(val _self: Foo) extends AnyVal {
  def size:Int = _self match { case Leaf(_) => 1; case Branch(l, r) => l.size + r.size }
}


This avoids boxing and permits you to introduce helpers, without the
danger you note of accidentally introducing public methods in Foo.

Frankly, though, I have to wonder why you want OO-style methods at all,
at this point.  You seem to be following the [entirely sensible and
advantageous] design style of the F# standard library, providing
functions to operate on data structures (e.g. List), even though member
methods are available as a design feature.

-- 
Stephen Compall
"^aCollection allSatisfy: [:each|aCondition]": less is better

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