I've decided to start writing a weekly post about the bugs I encounter, either in my personal code or at work.  Part of this is to just share tips/tricks... part of this is to share lols.

Most of my bugs will likely be in Scala code.  For those unfamiliar, Scala is a JVM-like language that adds in functional programming.  Basically, it's Java + Python.

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This week's bug was a fun one.  So, we have this object (a singleton instance of a class), and it has a bunch of properties.  It also has an "All" property which is a set of them.  We'll say these properties represent languages:

object Languages {
 val All = Set(Java, Python, CPlusPlus)

 val Java = "java"
 val Python = "python"
 val CPlusPlus = "c++"
}

Can you spot the bug?  Yes, this does compile, so it's not that...

If I did this, you'd find it real quick:
scala> object Languages {
    | val All = Set(Java, Python, CPlusPlus)
    |
    | val Java = "java"
    | val Python = "python"
    | val CPlusPlus = "c++"
    | }
defined module Languages

scala> Languages.All
res0: scala.collection.immutable.Set[java.lang.String] = Set(null)
wtf.  Why is it a set of null?!  Well, as it turned out (after 4 hours of scouring through a ton of code built on top of this simple object), it's because Scala objects and classes evaluate those out of order... if I had made the vals below lazy (evaluated on call; not on defining the class) it'd have worked properly.  Sadly, that has a memory overhead.

The real solution is just to put All at the bottom of Languages:
object Languages {
  val Java = "java"
  val Python = "python"
  val CPlusPlus = "c++"

  val All = Set(Java, Python, CPlusPlus)
}

And boom.  It worked:
scala> object Languages {
    |   val Java = "java"
    |   val Python = "python"
    |   val CPlusPlus = "c++"
    |
    |   val All = Set(Java, Python, CPlusPlus)
    | }
defined module Languages

scala> Languages.All
res0: scala.collection.immutable.Set[java.lang.String] = Set(java, python, c++)

So, yeah, this was just an annoying bug that cost me almost an entire day.  Figured I'd share my annoyances.