Scala `+` functor defined on Int maps from Short to Int?

Question

I'm writing a custom tree/graph container for smaller serialisation, and i'm getting a compiler error, apparently adding something to a short produces an Int.

def traverse(tree : Tree, level : Short = 0, pos : Short = 0) {
  buf(level) += tree
  var pos : Short = -1
  tree.getChildrenAsList.iterator().foreach { z=>
    pos += 1
    traverse(z, level + 1 ,pos)
  }
}

[error] /Users/hassan/code/scala/avro/src/main/scala/edu/hsyed/nlp/MyTree.scala:33: type mismatch;
[error]  found   : Int
[error]  required: Short
[error]         traverse(z, level + 1 ,pos)

I'm having to do this, which seems a bit unusual because of scala's impressive type deduction :

def traverse(tree : Tree, level : Short = 0, pos : Short = 0) {
      buf(level) += tree
      var pos : Short = -1
      tree.getChildrenAsList.iterator().foreach { z=>
        pos = (pos + 1).toShort
        traverse(z, (level + 1).toShort ,pos)
      }
    }

edit

Aah the question is, how do I get around this ?

This look suspiciously like a consequence of [Java's primitive numerical shenanigans](http://stackoverflow.com/questions/2720738/java-short-and-casting). — mikołak, Feb 18 '14 at 12:19

senia · Accepted Answer · 2014-02-18T13:16:12.660

You could create additional method for sum of Short.

There is already such methods in scala in Numeric type class:

import scala.math.Numeric

val ns = implicitly[Numeric[Short]]
val s = 1: Short

ns.plus(s, s)
// Short = 2

ns.times(3, 2)
// Short = 6

In case you can abstract over Short type you could use + method from Numeric.Implicits like this:

import Numeric.Implicits._
def traverse[T: Numeric](tree: Tree[T], level: T, pos: T) {
  val one = implicitly[Numeric[T]].one

  buf(level) += tree
  var pos: T = -one
  tree.getChildrenAsList.iterator().foreach { z =>
    pos += one
    traverse(z, level + one, pos)
  }
}

You could also use |+| from scalaz as replacement for +:

import scalaz._, Scalaz._

s |+| s
// Short = 2

Very high quality answer, the second section of code especially. The first part alone would not have sufficed in getting the intention/use-case of the Numeric class across. It is also a good bridge for why scalaz, and further why monoids since you showed the `.one` method. At some point I need to understand what `implicitly` is meant for :D — Hassan Syed, Feb 18 '14 at 13:14

Scala `+` functor defined on Int maps from Short to Int?

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