min/max of collections containing NaN (handling incomparability in ordering)

Question

I just ran into a nasty bug as a result of the following behavior:

scala> List(1.0, 2.0, 3.0, Double.NaN).min
res1: Double = NaN

scala> List(1.0, 2.0, 3.0         


        

Answer 1

This answer is simply for explaining the issue, @monkjack's answer probably provides the best practical solution.

Now since Scala offers the possibility to implicitly pass such an ordering, isn't it a natural desire to pass an ordering, which can handle "incomparability" according to our demands

Ordering in Scala represents only total orderings, i.e. ones where all elements are comparable. There is a PartialOrdering[T]: http://www.scala-lang.org/api/2.10.3/index.html#scala.math.PartialOrdering, but there are a couple of problems:

1. It is not actually used anywhere in the standard library.

2. If you try to implement max/maxBy/etc. which take PartialOrdering, you'll quickly see that it isn't generally possible except in cases like Float/Double where you have some elements which aren't comparable with anything and all the rest are comparable with each other (and you can decide to just ignore the incomparable elements).