Question

I’m currently learning Scala by working through the “Programming in Scala” book. So far, there have been nice explanations for everything that looks weird (from a Java programmer’s perspective), but this one example using a Stream to generate the Fibonacci sequence leaves me kind of puzzled:

def fibFrom(a: Int, b: Int): Stream[Int] =
  a #:: fibFrom(b, a + b)

How is the construction of the Stream done? Of course the #:: operator is somehow responsible for that. I understand that since it ends in :, it is right-associative, but that does not explain the creation of the Stream. I guess it is implicitly translated to a constructor somehow but I don’t see why and how exactly.

I’ve already looked for answers in Predef.scala and LowPriorityImplicits.scala but no luck so far.

Can anyone enlighten me?

Answer 1

It is right associative so it works as a method on the right argument:

fibFrom(b, a + b).#::(a)

At least that is what it tries to do syntactically. Stream[Int] does not have such a method. Luckily though, object Stream has an implicit to some class ConsWrapper which has this method (code).

So, what you get after implicit resolution is this:

immutable.this.Stream.consWrapper(fibFrom(b, a + b)).#::(a)