"Constructor cannot be instantiated to expected type" when defining streams in Scala - scala

Question

I tried to implement a StreamInt (without using generic types) and got an error message from REPL
<console>:29: error: constructor cannot be instantiated to expected type;
found : Cons
required: StreamInt.type
case Cons(hd,tl)=> if (n==0) hd else tl().nth(n-1)
The code is pasted below. Anyone can show me how to get it right?
trait StreamInt
case object Empty extends StreamInt
case class Cons (hd: Int, tl: ()=>StreamInt) extends StreamInt
object StreamInt{
def cons(hd:Int, tl: => StreamInt):StreamInt = {
val head=hd
lazy val tail=tl
Cons(head, ()=>tail)
}
def empty:StreamInt = Empty
def apply(as: Int*):StreamInt={
if (as.isEmpty) empty
else cons(as.head, apply(as.tail: _*))
}
def nth(n:Int):Int= this match {
case Cons(hd,tl)=> if (n==0) hd else tl().nth(n-1)
case _ => throw new Exception("out of bound!")
}
}

Answer 1

this match {
That line means you are matching the containing object, in your case that is object StreamInt. That (companion) object can never be a Cons cell. You probably want to have the method nth on the class or trait StreamInt:
sealed trait StreamInt {
def nth(n:Int):Int= this match {
case Cons(hd,tl)=> if (n==0) hd else tl().nth(n-1)
case Empty => throw new Exception("out of bound!")
}
}
case object Empty extends StreamInt
case class Cons (hd: Int, tl: ()=>StreamInt) extends StreamInt
object StreamInt{
def cons(hd:Int, tl: => StreamInt):StreamInt = {
val head=hd
lazy val tail=tl
Cons(head, ()=>tail)
}
def empty:StreamInt = Empty
def apply(as: Int*):StreamInt={
if (as.isEmpty) empty
else cons(as.head, apply(as.tail: _*))
}
}
Now this refers to the trait StreamInt which may indeed by a Con or Empty.
I also added the sealed modifier, that ensures that the compiler can warn you if you miss a particular case when using pattern matching.