Scala wrong forward reference - scala

Question

I am working through some of the exercises in: Functional Programming in Scala specifically problem 5.2. The issue is that with the following code which I have pieced together from the answer key.
sealed trait Stream[+A]
{
def take(n: Int): Stream[A] = this match {
case Cons(hs, ts) if n > 1 => cons(h(), t().take(n - 1))
case Cons(hs, _) if n == 1 => cons(h(), empty)
case _ => empty
}
}
case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]
object Stream{
def cons[A](hd: => A, tl: => Stream[A]): Stream[A] = {
lazy val head = hd
lazy val tail = tl
Cons(() => head , () => tail)
}
def empty[A]: Stream[A] = Empty
def apply[A](as: A*): Stream[A] =
if (as.isEmpty) empty
else cons(as.head, apply(as.tail: _*))
}
I get the following in the REPL:
<console>:10: error: not found: type A
def take(n: Int): Stream[A] = this match {
^
<console>:11: error: not found: value Cons
case Cons(hs, ts) if n > 1 => cons(h(), t().take(n - 1))
^
<console>:11: error: not found: value cons
case Cons(hs, ts) if n > 1 => cons(h(), t().take(n - 1))
^
<console>:12: error: not found: value Cons
case Cons(hs, _) if n == 1 => cons(h(), empty)
^
<console>:12: error: not found: value cons
case Cons(hs, _) if n == 1 => cons(h(), empty)
^
<console>:13: error: not found: value empty
case _ => empty
^

Answer 1

You have 2 problems in this code:
Not explicitly specifying that the empty and cons methods are located in the companion object Stream
To fix this you need to either import Stream._ into your class:
sealed trait Stream[+A] {
import Stream._
def take(n: Int): Stream[A] = this match {
case Cons(hs, ts) if n > 1 => cons(hs(), ts().take(n - 1))
case Cons(hs, _) if n == 1 => cons(hs(), empty)
case _ => empty
}
}
Or you need to explicitly specify it:
sealed trait Stream[+A] {
def take(n: Int): Stream[A] = this match {
case Cons(hs, ts) if n > 1 => Stream.cons(hs(), ts().take(n - 1))
case Cons(hs, _) if n == 1 => Stream.cons(hs(), Stream.empty)
case _ => Stream.empty
}
}
Using the variable names of t and h that are in case class Cons instead of the bound variables of hs and ts.
When you do this:
case Cons(hs, ts) if n > 1 => Stream.cons(hs(), ts().take(n - 1))
case Cons(hs, _) if n == 1 => Stream.cons(hs(), Stream.empty)
You are saying that you want to extract the case class parameters as hs and ts respectively and use them in the next code block. It does not matter if they were called h and t in the case class, they will be assigned the names you specify in the match.
Fixing these two issues and your code should compile (I personally tested it with Scala 2.11.5 and Java 1.7, but I don't think it should matter):
sealed trait Stream[+A] {
def take(n: Int): Stream[A] = this match {
case Cons(hs, ts) if n > 1 => Stream.cons(hs(), ts().take(n - 1))
case Cons(hs, _) if n == 1 => Stream.cons(hs(), Stream.empty)
case _ => Stream.empty
}
}
case object Empty extends Stream[Nothing]
case class Cons[+A](h: () => A, t: () => Stream[A]) extends Stream[A]
object Stream{
def cons[A](hd: => A, tl: => Stream[A]): Stream[A] = {
lazy val head = hd
lazy val tail = tl
Cons(() => head , () => tail)
}
def empty[A]: Stream[A] = Empty
def apply[A](as: A*): Stream[A] =
if (as.isEmpty) empty
else cons(as.head, apply(as.tail: _*))
}