Scala type mismatch, cannot resolve symbol A, Pattern type is incompatible with expected type

Question

I'm working on writing the Stream class in Chapter 5 of Functional Programming in Scala, I know the solutions are online, but it's not helping me. I faced the same issue with the previous Chapter writing the List class.

I got so frustrated I actually COPY PASTED from the solution to my Scala worksheet and still the same issue.

I thought maybe it's because of the name (there's already a List and Stream), doesn't seem like a smart idea to name them like this, so I changed it, didn't help.

Maybe it's something to do with Intellij (I'm using IntelliJ IDEA), I'm doing the exercises on the Scala Worksheets. But I can't find anything about this issue in relation to IDEs.

Here is what I have so far:

sealed trait StreamRED[+A]
case object Empty extends StreamRED[Nothing]
case class Cons[+A](h: () => A, t: () => StreamRED[A]) extends StreamRED[A]

object StreamRED {
  def cons[A](hd: => A, tl: => StreamRED[A]): StreamRED[A] = {
    lazy val head = hd
    lazy val tail = tl
    Cons(() => head, () => tail)
  }
  def empty[A]: StreamRED[A] = Empty

  def apply[A](as: A*): StreamRED[A] =
    if (as.isEmpty) empty else cons(as.head, apply(as.tail: _*))

  def headOption: Option[A] = this match {
    case Empty => None
    case Cons(h,t) => Some(h())
  }

  def toList: List[A] = {
    @annotation.tailrec
    def go(s: StreamRED[A], acc: List[A]): List[A] = s match {
      case Cons(h,t) => go(t(), h() :: acc)
      case _ => acc
    }
    go(this, List()).reverse
  }
}

I get the following errors:

"Cannot resolve symbol A" on the A in Option[A] (in headOption method) and List[A] and StreamRED[A] (in toList)

"Type mismatch. Required: StreamRED[Any], Found: StreamRED.type" on the this in toList.

"Pattern type is incompatible with expected type, found: Empty.type, required: StreamRED.type" on the Empty in headOption.

New to Scala, new to IntelliJ, new to statically typed languages, new to FP. Any explanations and recommendations for good reading materials much appreciated.

Markus Appel Markus Appel · Accepted Answer · 2020-01-08T14:27:57

The two functions toList and headOption cannot be defined in the companion object of StreamRED.

If you define them directly in the trait it works:


sealed trait StreamRED[+A] {

  def headOption: Option[A] = this match {
    case Empty => None
    case Cons(h,t) => Some(h())
  }

 def toList: List[A] = {
    @annotation.tailrec
    def go(s: StreamRED[A], acc: List[A]): List[A] = s match {
      case Cons(h,t) => go(t(), h() :: acc)
      case _ => acc
    }
    go(this, List()).reverse
  } 
}

case object Empty extends StreamRED[Nothing]
case class Cons[+A](h: () => A, t: () => StreamRED[A]) extends StreamRED[A]

object StreamRED {
  def cons[A](hd: => A, tl: => StreamRED[A]): StreamRED[A] = {
    lazy val head = hd
    lazy val tail = tl
    Cons(() => head, () => tail)
  }
  def empty[A]: StreamRED[A] = Empty

  def apply[A](as: A*): StreamRED[A] =
    if (as.isEmpty) empty else cons(as.head, apply(as.tail: _*))
}

A word of warning: Pattern matching on this is feels to me like bad practice. You know exactly what this is. Implement the functions in Empty and Cons instead.

Do this instead:

sealed trait StreamRED[+A] {
  def headOption: Option[A]
  def toList: List[A]
}

case object Empty extends StreamRED[Nothing] {
  def headOption: Option[Nothing] = None
  def toList: List[Nothing] = List()
}


case class Cons[+A](h: () => A, t: () => StreamRED[A]) extends StreamRED[A] {
  def headOption: Option[A] = Some(h())
  def toList: List[A] = h() +: t().toList
}

object StreamRED {
  def cons[A](hd: => A, tl: => StreamRED[A]): StreamRED[A] = {
    lazy val head = hd
    lazy val tail = tl
    Cons(() => head, () => tail)
  }
  def empty[A]: StreamRED[A] = Empty

  def apply[A](as: A*): StreamRED[A] =
    if (as.isEmpty) empty else cons(as.head, apply(as.tail: _*))
}

Scala type mismatch, cannot resolve symbol A, Pattern type is incompatible with expected type

1 Answers