Scala 2.10, Double.isNaN, and boxing

Question

In Scala 2.10, is someDouble.isNaN expected to box? Running my code calling .isNaN through a decompiler, I still see telltale calls to double2Double in my code. Given the new AnyVal work in 2.10, I'd expect it to be no worse than java.lang.Double.isNaN(someDouble) at runtime with no spurious allocations. Am I missing something?

Zaragoza

Answer 1

Unfortunately, isNaN is a method on java.lang.Double, and it is essential to have an implicit conversion to java.lang.Double, so the Scala RichDouble value class cannot reimplement isNaN to be fast, and when you use isNaN you box to java.lang.Double.

Since this leaves only slow or awkward ways to test for NaN, I define

implicit class RicherDouble(val d: Double) extends AnyVal {
  def nan = java.lang.Double.isNaN(d)
}

and then I can just use .nan to check.

Three

Why should this be any faster than boxing the double? – Curium

@Curium - Because this "boxes" to a value class (that's the extends AnyVal part), which isn't actually created unless you need to pass it as an object or generic or put it in an array. If you just use it (like you are here), Scala emits bytecode that is just a method call, since there's actually nothing that requires a real object to exist. – Three

Couldn't there be a higher-priority implicit to your RicherDouble, with the same isNaN name on it? Or if not, something of the same priority to fail with ambiguity to remind yourself not to use that method in tight loops. – Zaragoza

@MyseriousDan - I do actually define the isNaN method also to get an ambiguous implicit conversion warning. As for making it higher priority, you can't do that without changing the library, and I'm not sure whether the necessary change would break anything. – Three

@RexKerr Nice. Missed the AnyVal there. – Curium

@RexKerr do you have any particular possible breakage in mind? I could fix this with an additional layer of predef pastry. – Fetich

@extempore - I don't, I'm just extremely wary of moving priorities around in something so incredibly heavily used. – Three