Error while implementing take for a Stream

Question

I was trying to solve Exercise 2 from Functional Programming in Scala. The question is as follows:

EXERCISE 2: Write a function take for returning the first n elements of a Stream. def take(n: Int): Stream[A]

My solution is as follows:

    import Stream._
    trait Stream[+A]{
            def uncons:Option[(A,Stream[A])]
            def isEmpty:Boolean = uncons.isEmpty
            def toList:List[A] = {
                    val listBuffer = new collection.mutable.ListBuffer[A]
                    @annotation.tailrec
                    def go(str:Stream[A]):List[A] = str uncons match {
                            case Some((a,tail)) => listBuffer += a;go(tail)
                            case _ => listBuffer.toList
                    }
                    go(this)
            }

            def take(n:Int):Stream[A] = uncons match {
                    case Some((hd,tl)) if (n > 0) => cons(hd,tl.take(n-1))
                    case _ => Stream()
            }
    }

   object Stream{
        def empty[A]:Stream[A] = new Stream[A]{def uncons = None}
        def cons[A](hd: => A,tl: => Stream[A]):Stream[A] = new Stream[A]{
                lazy val uncons = Some((hd,tl))
        }
        def apply[A](as: A*):Stream[A] = {
                if(as.isEmpty) empty else
                        cons(as.head,apply(as.tail: _ *))
        }
}

I store this as Stream2.scala and then from the REPL I execute the following:

:load Stream2.scala

When the REPL tries to load my script, it barfs with the following errors:

scala> :load Stream2.scala

Loading Stream2.scala...
import Stream._
<console>:24: error: type mismatch;
 found   : Stream[A]
 required: scala.collection.immutable.Stream[?]
            case Some((hd,tl)) if (n > 0) => cons(hd,tl.take(n-1))
                                                                ^
<console>:25: error: type mismatch;
 found   : scala.collection.immutable.Stream[Nothing]
 required: Stream[A]
            case _ => Stream()
                                ^
<console>:11: error: object creation impossible, since method tailDefined in class Stream of type => Boolean is not defined
        def empty[A]:Stream[A] = new Stream[A]{def uncons = None}
                                     ^
<console>:12: error: object creation impossible, since method tailDefined in class Stream of type => Boolean is not defined
        def cons[A](hd: => A,tl: => Stream[A]):Stream[A] = new Stream[A]{

Can somebody point out what might be going wrong here?

Try this outside of the REPL - compile the code using scalac Stream2.scala. It can be that the REPL doesn't connect the companion object to the Stream trait. Also there seems to be some interference from the standard library Stream (which shares the same name as your Stream). Compiling outside the REPL may provide longer error messages. — huynhjl
Or coming to a repl near you, perhaps, :paste file, issues.scala-lang.org/browse/SI-4684 — som-snytt

4lex1v 4lex1v · Accepted Answer · 2013-07-01T07:54:30

Just place import statement under the Stream trait. It doesn't work cause for Scala compiler you are importing scala.collection.immutable.Stream, but not your companion object. And, as it was sad in the comments use :paste in console, but paste it in as a hole code, otherwise it won't be a companion object to your trait

Error while implementing take for a Stream

1 Answers