Scala generics: type mismatch with folding

Question

New to scala. Trying to understand why scala compiler is not happy about the below:

sealed trait LinkedList[A] {
  def fold[B](end: B)(func: (A, B) => B): B =
    this match {
      case End() => end
      case Pair(hd, tl) => func(hd, tl.fold(end)(func))
    }

  def sum: Int =
    fold[Int](0){(hd, tl) => hd + tl}
}

final case class Pair[A](head: A, tail: LinkedList[A]) extends LinkedList[A]
final case class End[A]() extends LinkedList[A]

object Foo extends App {
  val example = Pair(1, Pair(2, Pair(3, End())))
  println(example.sum)
}

Getting this error:

Error:(10, 35) type mismatch;
 found   : Int
 required: String
    fold[Int](0){(hd, tl) => hd + tl}

How is String being inferred here?

Please help.

Answer 1

For a general A , usual "addition" is not defined. So instead, it implicitly converts A into String , and uses the + that concatenates String s. A quick and dirty workaround would be:

def sum(implicit i: A =:= Int): Int = fold[Int](0){(hd, tl) => i(hd) + tl}

This would make sum available only if A is Int . A somewhat more systematic approach would be to use Numeric typeclass, just like the method in the standard library (unfold "use case" and "Full signature").