Scala variable scope in object

Question

I'm looking at http://hseeberger.wordpress.com/2010/11/25/introduction-to-category-theory-in-scala/ and there's a bit of code that I can't understand how it works:

object Functor {

  def fmap[A, B, F[_]](as: F[A])(f: A => B)(implicit functor: Functor[F]): F[B] =
    functor.fmap(as)(f)

  implicit object ListFunctor extends Functor[List] {
    def fmap[A, B](f: A => B): List[A] => List[B] =
      as => as map f
  }
}

Specifically, how is ListFunctor.fmap accessing as when the definition of as is in the scope of Functor.fmap and (as far as I can tell) inaccessible to ListFunctor.fmap?

(related with a twist) There's a previous iteration to the above code that's defined:

trait Functor[F[_]] extends GenericFunctor[Function, Function, F] {
  final def fmap[A, B](as: F[A])(f: A => B): F[B] =
    fmap(f)(as)
}

object ListFunctor extends Functor[List] {
  def fmap[A, B](f: A => B): List[A] => List[B] = as => as map f
}

But again, as seems to be magically accessible to ListFunctor. I guess if I understand one of these, I'll understand the other.

Michał Politowski · Accepted Answer · 2012-06-21 21:52:46Z

3

It is not the same as, just the same name is (probably not quite) accidentally used in two places. The as => as map f is a function definition, and the as before the arrow is a declaration of a parameter for this very function.

It would be completely equivalent if written as x => x map f.

answered Jun 21, 2012 at 21:52

Michał Politowski

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