Get companion object of type parameter

[Mitchell Hashimoto]

Hello, I'm trying to do something like the following:

class Utility[T] {
def doSomething(): T {
T.staticMethod()
}
}

Where T may be some type like the following:

object Foo {
def staticMethod() = new Foo()
}

I realize various weaknesses of the above but hopefully you get the point :)

I'm probably doing something horribly wrong but I'm hoping to be at
least pointed in the right direction from this list. How would you do
the above?

Thanks,
Mitchell

[Lex]

You could add getCompanion() to your classes and forward the call.

Another way is to make your companion object implicit and inject it in the Utility class via constructor:

trait OOHack[T] {

  def staticMethod()

}

class Foo

implict object Foo with OOHack[Foo] { ... }

class Utility[T] (implicit val anOOHack: OOHack[T]) {
 def doSomething() {
   anOOHack.staticMethod()

[Alexey Romanov]

I don't think it's possible (but would be happy to learn I was wrong).
T could extend a template with a "companion" method, as in
http://www.scala-lang.org/api/current/scala/collection/generic/GenericTraversableTemplate.html,
but even then you would first need an object of the type T to call
this method on.

Yours, Alexey Romanov

[Naftoli Gugenheim]

Do you mean companion object as used in the scala language, or do you just need types to point you to a factory object?

If the former, do you require top-level types? Either way, you could probably use reflection to find it, although I don't know if all the rules or how likely they are to change.

If the latter, one option is something like (not 100% sure it works):

class Utility[T](implicit factory: Factory[T]) {

  def doSomething: T = factory.method

}

trait Factory[T] {

  def method: T

}

// somewhere else

trait Foo

object Foo {

  implicit def factory: Factory[T] = new Factory[T] { def method = new Foo {} }

}

//Would this work? (No idea)

implicit object Foo extends Factory[T] {

  def method = new Foo

[Mitchell Hashimoto]

Naftoli,

On Sun, Mar 6, 2011 at 5:57 PM, Naftoli Gugenheim <nafto...@gmail.com> wrote:
> Do you mean companion object as used in the scala language, or do you just
> need types to point you to a factory object?

I suppose you're right, I just want access to call static methods on the class.

I guess it would've helped if I explained my problem, and maybe
someone can recommend a more scala-like way of solving it. I have a
bunch of classes which are supposed to be instantiated by converting
from a Jsoup Element class. So I wanted to enforce that all of the
classes have a companion object which has a method like "fromElement"
to convert to an instance of the class.

I'm still not sure how to do this.

> If the former, do you require top-level types? Either way, you could
> probably use reflection to find it, although I don't know if all the rules
> or how likely they are to change.
> If the latter, one option is something like (not 100% sure it works):
> class Utility[T](implicit factory: Factory[T]) {
>   def doSomething: T = factory.method
> }
> trait Factory[T] {
>   def method: T
> }
> // somewhere else
> trait Foo
> object Foo {
>   implicit def factory: Factory[T] = new Factory[T] { def method = new Foo
> {} }
> }
> //Would this work? (No idea)
> implicit object Foo extends Factory[T] {
>   def method = new Foo
> }

This idea looks interesting ,but I'm still hoping there is a more
concise way to solve my problem above :) Either way, I'll take a
closer look at what you've given me here.

Thanks,
Mitchell

[Kevin Wright]

On 7 Mar 2011 03:37, "Mitchell Hashimoto" <mitchell....@gmail.com> wrote:
>
> Naftoli,
>
> On Sun, Mar 6, 2011 at 5:57 PM, Naftoli Gugenheim <nafto...@gmail.com> wrote:
> > Do you mean companion object as used in the scala language, or do you just
> > need types to point you to a factory object?
>
> I suppose you're right, I just want access to call static methods on the class.
>
> I guess it would've helped if I explained my problem, and maybe
> someone can recommend a more scala-like way of solving it. I have a
> bunch of classes which are supposed to be instantiated by converting
> from a Jsoup Element class. So I wanted to enforce that all of the
> classes have a companion object which has a method like "fromElement"
> to convert to an instance of the class.
>
> I'm still not sure how to do this.
>

It's a classic use case for type classes. Look at how Numeric, for example, enforces that certain types have a `+` operation.