NaN's are extremely useful for dealing with missing data as one can
avoid large numbers of existence checks. For example:
a,b,c = GetFromDatabase("a", "b", "c")
d = a + b/c
if d == d then return "invalid" end
is much clearer than putting a lot of if-then checks on the
GetFromDatabase results provided that GetFromDatabase() can return
NaN. Using zero for missing data is problematic as division won't
return a consistent value and it is often a legit value in and of
itself. NaNs propagate nicely through arithmetical calculations and
have many of the semantics of SQL NULL values. Putting aside all the
divisions of opinion in the SQL community about the validity of having
NULLs in the first place, I find that NaNs make numeric code in the
presence of missing values easier to write and to understand. Clearly
code that expects the NaN behaviour will not run if Lua is compiled
with integer numbers, but then again most of the math library will not
either... Given that math.pi is provided, it would be nice to round
out the collection of standard constants.
