Use Cons operator for a custom trait - scala

Question

I have a MyStream-trait:
trait MyStream[+A] {
def uncons: Option[(A, MyStream[A])]
def isEmpty: Boolean = uncons.isEmpty
}
object MyStream {
def empty[A]: MyStream[A] =
new MyStream[A] { def uncons = None }
def cons[A](hd: => A, tl: => MyStream[A]): MyStream[A] =
new MyStream[A] { lazy val uncons = Some((hd, tl)) }
def apply[A](as: A*): MyStream[A] = if (as.isEmpty) empty
else cons(as.head, apply(as.tail: _*))
}
How can I use the uncons operator for pattern matching like:
def takeWhile(f: A => Boolean): MyStream[A] = this match {
case uncons(h,t) if f(h()) => cons(h(), t() takeWhile f)
case _ => empty
}
I am very new to Scala so I need a little help here.
Thanks!

Answer 1

How about this:
trait MyStream[+A] {
def uncons: Option[(A, MyStream[A])]
def isEmpty: Boolean = uncons.isEmpty
def takeWhile(f: A => Boolean): MyStream[A] = this match {
case MyStream(h, t) if f(h) => MyStream.cons(h, t takeWhile f)
case _ => MyStream.empty
}
#tailrec
final def foldLeft[B](z: B)(op: (B, A) => B): B =
this match {
case MyStream(h, t) => t.foldLeft(op(z, h))(op)
case _ => z
}
override def toString = this.foldLeft("") { case (acc, x) => acc + x }
}
object MyStream {
def empty[A]: MyStream[A] =
new MyStream[A] {
def uncons = None
}
def cons[A](hd: => A, tl: => MyStream[A]): MyStream[A] =
new MyStream[A] {
lazy val uncons = Some((hd, tl))
}
def apply[A](as: A*): MyStream[A] =
if (as.isEmpty) empty
else cons(as.head, apply(as.tail: _*))
def unapply[A](stream: MyStream[A]) = stream.uncons
}
object TestMyStream extends App {
import MyStream._
val s = cons(1, cons(2, cons(3, empty)))
println("All: " + s)
println("Take < 3: " + s.takeWhile(_ < 3))
}
prints:
All: 123
Take < 3: 12