playing with NA for Julia

NA.j

type _NA
end

macro NA() # for easier typing!
    _NA()
end

NumberData = Union(_NA, Number)
StringData = Union(_NA, String)

# is there a way to avoid having to write down rules like this for every
# basic data type?
promote_rule(::Type{_NA}, ::Type{Int64}) = NumberData
promote_rule(::Type{_NA}, ::Type{Float64}) = NumberData
promote_rule(::Type{_NA}, ::Type{ASCIIString}) = StringData

# and likewise?
promote_rule(::Type{NumberData}, ::Type{Int64}) = NumberData
promote_rule(::Type{NumberData}, ::Type{Float64}) = NumberData
promote_rule(::Type{StringData}, ::Type{ASCIIString}) = StringData

# This works, but will we need to do it for EVERY operator?
+(x::_NA, y::Number) = @NA

show(na::_NA) = print("NA")

is_na(x) = typeof(x) == _NA


# try it out:
qq = ["asdf", "qwerty", @NA, "biff"]
qq[!map(is_na, qq)]
map(uc, qq) # fails -- will we have to change EVERY core function?

zz = [1:3, @NA, 5:6]
zz + 1
sqrt(zz) # fails -- same issue