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[fitsbits] Start of the CONTINUE keyword Public Comment Period
Perhaps the discussion from 1993 and 1994 could be included verbatim,
or linked by thread off the web page? I'd hate to hold a debate on a topic that was already batted around a dozen years ago. Is the registry page the entire text of the proposed convention? Like Tom said, you need to cover the obvious corner cases, at least by verbiage like "it is undefined what string is conveyed if a card ending is '&' is not followed by CONTINUE". 1) Couldn't the reordering question be dealt with via appending &NNN instead of &? That could resolve some of the corner cases, too, and you wouldn't need to reserve multiple CONT_NNN keywords. We should be leery of implicitly requiring that all FITS headers be now considered strictly ordered, when there is no practical way to enforce this. In particular, this has the side effect of making the interpolation of any new keyword questionable other than immediately before END. 2) What is the reason for not strictly requiring that such a continuation string end with ampersand-quote? What advantage is conveyed by permitting semantically null trailing whitespace that merely confuses the handling? Surely it should be incumbent on the author of such keywords to trim meaningless characters, not generations of readers yet unborn to dodge around them. 3) One suspects a number of conventions will depend on valueless keywords. The standard (2.1b) is rather convoluted on how this works: 5.1.2.2 Value Indicator (bytes 9–10) If this field contains the ASCII characters “=”, it indicates the presence of a value field associated with the keyword, unless it is a commentary keyword as defined in §5.4.2.4. If the value indicator is not present or if it is a commentary keyword then columns 9–80 may contain any ASCII text. Which basically just acts to preserve the legality of COMMENT, HISTORY and blank keywords to have an equals sign in column 9. 4) In general, the nature of a convention versus a standard is that others are perfectly within their rights to violate a convention for either good or not-so-good reasons. The behavior here seems simple, but that's where the risk lies. Nobody would be unsurprised that deleting the INHERIT keyword turns inheritance off - it's almost a feature. Delete the CHECKSUM keyword and you simply remove an assertion about the contents of the file. One can even productively use DATASUM and CHECKSUM independently. (I'll have to check if we made that legal.) On the other end of the spectrum, conventions like tiled image compression and foreign file encapsulation involve very specific FITS usage - a file must adhere to the standard and additionally must adhere to the complex requirements of the convention. The CONTINUE convention, however, is different in providing a more general capability that could be used, for instance, within any other convention. Deleting a CONTINUE card could in principle change the meaning of any header in arbitrary ways. I wonder if perhaps we shouldn't rather be discussing how best to add this capability to the core FITS standard. Whether convention or new feature of the standard, however, the loopholes should be closed before adoption. Rob |
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