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[fitsbits] Start of the CONTINUE keyword Public Comment Period
On Tue 2007/07/17 16:17:06 -0400, William Pence wrote in a message to: FITSBITS 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 |
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