[fitsbits] Start of the CONTINUE keyword Public Comment Period

[Mark Calabretta]

On Tue 2007/07/17 16:17:06 -0400, William Pence wrote
in a message to: FITSBITS <fitsbits at nrao.edu>

>Mark Calabretta wrote:
>>   1) The continuation character, '&', is redundant syntax.  As described
>>      in the prologue of fitshdr.h (from WCSLIB, as appended), it only
>>      indicates continuation if the following card is CONTINUE otherwise
>>      it must be interpreted literally.
>
>The redundancy is intentional in this case, and helps to avoid any
>possible confusion over whether the FITS writer really intended this
>convention to be used or not.

Bill,

The CONTINUE convention differs significantly from the others offered
for the registry because it defines basic functionality that people are
likely to want to use.  It will almost inevitably become a de facto
standard, if it hasn't already, or at least strongly influence the way
that continuation syntax might be standardised.  Consequently, I think
it is worth devoting some effort to settling on a syntax that we all
feel comfortable with.

My only concern with the convention as currently described is with the
use of the '&' character, which, to reiterate, is redundant syntax.
However, I don't advocate eliminating it.  Instead I suggest making it
optional in precisely the way described in the prologue of fitshdr.h
(previously appended).  In practice it can still serve the useful
function of "guarding" trailing blanks that are to be preserved in a
string value.  The prologue of fitshdr.h describes how this form of
CONTINUE-based continuation works in a parser that has been implemented.

Tying continuation to a particular data type, apart from being
unnecessary, must be unique amongst computer-based syntaxes.  The
argument that only string values are likely to be continued ignores
possible future syntaxes.  For example, record-valued keywords currently
proposed by/for WCS Paper IV might be better implemented by extending
the keyword syntax.  They could easily be long enough to require
continuation, and the continued portion could be a floating point value
or something else.

Against objections that have been raised relating to keyword ordering,
it should be borne in mind that once a header has been parsed into a
data structure it is largely inconsequential that the original ordering
might be lost; if the data structure is later retranslated into a FITS
header the keywords might appear in a different order (and with
different inline comments, decimal precision, etc.), but the CONTINUE
keywords would still have to be ordered correctly in the output header.
The same applies to COMMENT, HISTORY, or other order-dependent keywords.
Order must, and surely will be, preserved if and only if it matters.

Also, in response to recent comments, I should point out that Paper IV
advocates that the same record-valued keyword appear multiple times with
different keyvalues (and that the ordering is not significant).
Sect. 2.4.2 explains why.

Regards, Mark