posts: 085, draft

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+---
+postid: 085
+title: An XML parser for Common Lisp programs
+date: February 18, 2019
+author: Lucian Mogoșanu
+tags: tech, tmsr
+---
+
+This post is part of a [series][tmsr-schedule-i] of published
+[artifacts][intellectual-ownership] that (will) represent components for
+The Republic's RSS bot, [Feedbot][feedbot]. The idea behind this series
+is to grow Feedbot piece by piece[^1], starting from the smallest
+elements that [fit in head][fits-in-head], then using them as building
+blocks for the actual product, which will flow downstream from the
+[botworks][botworks] V tree.
+
+The first item to be published is S-XML, an XML parser written in Common
+Lisp. Both the name and the code have been lifted from files
+[published][s-xml] by one [nonperson][nonperson] known as Sven Van
+Caekenberghe, who, fortunately, wrote a library that is relatively small
+(around a thousand LoC), is organized so that it can be grasped in one
+sitting, and is known to work[^2]. Unfortunately though, as with most
+(all?) heathen programs encountered, this one isn't without warts. Thus,
+in addition to providing a patch, this article discusses the structure
+of S-XML and its current problems.
+
+The patch for S-XML is available in my
+[V source repository][s-xml-v]. Now, as to the library itself, it is
+structured as follows.
+
+S-XML contains two layers of abstraction: a. the core parsing code, that
+reads characters from a stream and returns XML elements, stored in
+`xml.lisp`; b. a series of wrappers over the parser that take the parser
+results and give them a particular structure; and c. an interface
+between (a) and (b), stored in `dom.lisp`. In fact, one could say that
+the layering goes exactly the other way around: the code in (b) provides
+a set of functions for the parser (a), while the parser takes a
+string/stream, reads it and calls the functions provided by (b) so it
+can decide what to do once it has all the required data, e.g. tags,
+attributes etc. The wrappers in (b) are stored in `xml-struct-dom.lisp`,
+`lxml-dom.lisp` and `sxml-dom.lisp`[^3].
+
+The advantage of this design is that it doesn't constrain the user to
+any particular DOM tree representation. Personally, I find this
+so-called feature to be entirely useless and not much of an advantage
+after all, as I don't have any plans to parse XML files into multiple
+tree formats, not even mentioning the fact that "doesn't constrain the
+user" is the perfect recipe for
+[hallucinated freedom][hallucinated-freedom], and not even discussing
+the extra lines of (mostly dead) code this adds. This
+[cleverness][clever] is more for its own sake than anything of
+substance, and thus eventually some hero or another will surgically
+excise this particular tumour.
+
+Until then, however, the thing works, so it's all the better to publish
+it than to wait for the moment when said hero gets off his or her
+ass. Until then, the more pressing matter for yours truly, and the next
+episode of this series, will involve publishing a small RSS/Atom parser
+based on S-XML.
+
+[^1]: This style, nowadays immediately recognizable by TMSR citizens as
+    the "FFA style", draws from [Asciilifeform][alf]'s
+    [Finite Field Arithmetic][loper-os] series.
+
+[^2]: It's been powering Feedbot for some months now.
+
+[^3]: Corresponding, respectively, to a defstruct-based format, Franz's
+    [LXML][lxml] format and the so-called [SXML][sxml]. The latter two
+    structure XML markup as S-expressions. Currently, Feedbot's RSS
+    parser uses LXML, for no reason in particular other than it being
+    the implicit option.
+
+[tmsr-schedule-i]: /posts/y05/082-tmsr-schedule-i.html#selection-63.114-63.159
+[intellectual-ownership]: /posts/y04/069-on-intellectual-ownership.html
+[feedbot]: http://btcbase.org/log-search?q=feedbot
+[alf]: http://wot.deedbot.org/17215D118B7239507FAFED98B98228A001ABFFC7.html
+[loper-os]: http://www.loper-os.org/?cat=49
+[fits-in-head]: http://btcbase.org/log-search?q=fits-in-head
+[botworks]: /posts/y05/080-botworks-regrind.html
+[s-xml]: https://archive.is/Ra6bA
+[nonperson]: http://btcbase.org/log-search?q=nonperson
+[s-xml-v]: http://lucian.mogosanu.ro/src/ TODO update this with actual dir
+[lxml]: https://web.archive.org/web/20080108082030/http://opensource.franz.com/xmlutils/xmlutils-dist/pxml.htm
+[sxml]: https://archive.is/Sl4Ev
+[hallucinated-freedom]: http://trilema.com/2017/the-practical-costs-of-hallucinated-freedom/
+[clever]: http://btcbase.org/log-search?q=clever