On Wed, 21 Jun 2006, Mark Taylor wrote:
> I can see that this can be justified in theoretical terms, but I must
> admit I've never thought much about the distinction and any software
> I've written has got on fine by using NaN (or a magic integer value)
> to cover both undefined and numerically indefinite. FITS BINTABLE
> and IMAGE seem to have managed without the distinction as well.
> Are there really compelling scenarios for this requirement?
In the world of relational databases there have long been disputes over how many different types of null should be supported. Since the set-theory basis of RDBMS actually becomes inconsistent in the face of even one type of null, there are many who think that they are all evil.
In practice, as Mark said astronomers have long managed with just one type of null in FITS files. Once the idea of having more than one type of null gets around it will be very hard to get agreement on where to stop, and I have seen little evidence of need for more than one. I think it would be more productive to work on a standard way of expressing the notion of upper (or lower) limits - something which is so far missing from most astronomical formats.
-- Clive Page Dept of Physics & Astronomy, University of Leicester, Tel +44 116 252 3551 Leicester, LE1 7RH, U.K. Fax +44 116 252 3311Received on 2006-06-21Z10:05:55