Incorrect filter Implementation in Chapter 3.3 (Exercise: Stream Combinators)

[AleAndForCode]

In Chapter 3.3, Exercise: Stream Combinators, the provided implementation of the filter method for the Stream trait seems incorrect.

In the current implementation, the tail method doesn’t correctly apply the filtering logic. It simply calls stream.tail, which doesn’t skip non-matching elements, causing infinite recursion or incorrect results.

Current Implementation:

def filter(pred: A => Boolean): Stream[A] = {
  val self = this
  new Stream[A] {
    def head: A = {
      def loop(stream: Stream[A]): A =
        if pred(stream.head) then stream.head
        else loop(stream.tail)

      loop(self)
    }

    def tail: Stream[A] = {
      def loop(stream: Stream[A]): Stream[A] =
        if pred(stream.head) then stream.tail
        else loop(stream.tail)

      loop(self)
    }
  }
}

Suggested Fix:
The tail method should continue filtering the stream.

def tail: Stream[A] = {
  def loop(stream: Stream[A]): Stream[A] =
    if (pred(stream.head)) stream.tail.filter(pred)
    else loop(stream.tail)
  loop(self)
}

Thanks for the great book!