Scala 2.8 CanBuildFrom

Following on from another question I asked, Scala 2.8 breakout, I wanted to understand a bit more about the Scala method TraversableLike[A].map whose signature is as follows:

def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Repr, B, That]): That

Notice a few things about this method:

• It takes a function turning each A in the traversable into a B.
• It returns That and takes an implicit argument of type CanBuildFrom[Repr, B, That].

I can call this as follows:

> val s: Set[Int] = List("Paris", "London").map(_.length)
s: Set[Int] Set(5,6)

What I cannot quite grasp is how the fact that That is bound to B (that is, it is some collection of B's) is being enforced by the compiler. The type parameters look to be independent of both the signature above and of the signature of the trait CanBuildFrom itself:

trait CanBuildFrom[-From, -Elem, +To]

How is the Scala compiler ensuring that That cannot be forced into something which does not make sense?

> val s: Set[String] = List("Paris", "London").map(_.length) //will not compile

How does the compiler decide what implicit CanBuildFrom objects are in scope for the call?