Cryptic pattern matching example

[Dinduks]

Hi,

Imagine someone who has never done Scala or a similar language before, visiting the scala-lang site. Then he stumbles upon this:

// Define a set of case classes for representing binary trees.
sealed abstract class Tree[+A]
case class Node[+A](elem: A, left: Tree[A], right: Tree[A]) extends Tree[A]
case object Leaf extends Tree[Nothing]

// Return the in-order traversal sequence of a given tree.
def inOrder[A](t: Tree[A]): List[A] = t match {
  case Node(e, l, r) => inOrder(l) ::: List(e) ::: inOrder(r)
  case Leaf          => List()
}

Pattern matching can be explained more easily, with classes or lists for example.

That example could be changed to something similar to:

list match {
   case Nil => "empty"
   case 'a' :: tail => "starting by 'a'"
   case (head:Int) :: _ if head > 3 => "starting by an int greater than 3"
   case (head:Int) :: _ => "starting by an int"
   case _ => "whatever"
}

Taken from: http://matt.aimonetti.net/posts/2012/09/20/what-is-scala-pattern-matching/

[heathermiller]

Thanks a lot for your input. You're right that we can probably simplify that example. Though the issue with using just lists means that we miss out on the opportunity to introduce case classes in our pattern matching example, which should be introduced alongside of the concept of pattern matching in Scala.

I'd also side with the current proposition of this tree/ordering example only because it's a real, non-trivial example. The thing about simple pattern matching on an arbitrary list (with no goal in mind) is that it's a bit context-less and like a toy example. I like that we can show an interesting actual solution in so few lines of code in Scala. Not sure if I like the complex look of the type parameters though :(

[sjrd]

Yes, thanks for your input. Funny that this code example receives the first comments: it was the first one we were happy with ^^

I think introducing case classes is important for this example. But I agree it can be too complex.

Maybe we can simplify it somewhat just by removing the type parameters. Just make it an IntTree.

[heathermiller]

Resolved in #94