{"id":387,"date":"2011-12-13T12:32:00","date_gmt":"2011-12-13T17:32:00","guid":{"rendered":"http:\/\/irreal.org\/blog\/?p=387"},"modified":"2011-12-13T12:40:09","modified_gmt":"2011-12-13T17:40:09","slug":"converting-s-expressions-to-xml-in-emacs","status":"publish","type":"post","link":"https:\/\/irreal.org\/blog\/?p=387","title":{"rendered":"Converting S-Expressions To XML In Emacs"},"content":{"rendered":"\n

My last<\/a> two<\/a> posts mentioned the ease with which s-expressions can be converted to XML and vice versa. Out of curiosity, I decided to write an sexpr to XML function in Elisp. <\/p>\n

The implementation consists of two functions. The first, sexpr->xml<\/code>, does the actual conversion, while the second, convert-to-xml<\/code>, takes care of Emacs bookkeeping details and acts as a driver. For convenience, convert-to-xml<\/code> takes an sexpr as its input but it would be easy to have it operate on a region or even a whole buffer. <\/p>\n

We’ll use the example sexpr from Yegge’s The Emacs Problem<\/a> post. Note that, again for simplicity, I am dealing with only a single record but everything would work exactly the same if there were multiple records wrapped in a '(log …)<\/code> sexpr. <\/p>\n

'(record\n  (date \"2005-02-21T18:57:39\"<\/span>)\n  (millis 1109041059800)\n  (sequence 1)\n  (logger nil)\n  (level 'SEVERE)\n  (class \"java.util.logging.LogManager$RootLogger\"<\/span>)\n  (method 'log)\n  (thread 10)\n  (message \"A very very bad thing has happened!\"<\/span>)\n  (exception\n    (message \"java.lang.Exception\"<\/span>)\n    (frame\n      (class \"logtest\"<\/span>)\n      (method 'main)\n      (line 30))))\n<\/pre>\n
 The converter is very simple. <\/p>\n
1:  <\/span>(defun<\/span> sexpr->xml<\/span> (sexpr)\n2:  <\/span>  (let<\/span> ((tag (car sexpr)))\n3:  <\/span>    (princ (format \"<%s>\"<\/span> tag))\n4:  <\/span>    (dolist<\/span> (o (cdr sexpr))\n5:  <\/span>      (if<\/span> (atom o)\n6:  <\/span>          (princ (format \"%s \"<\/span> o))\n7:  <\/span>        (sexpr->xml o)))\n8:  <\/span>    (princ (format \"<\/%s>\"<\/span> tag))))\n<\/pre>\n
 It’s passed an sexpr whose first symbol is the XML tag. That gets saved away in the let<\/code> on line 2 and printed as the opening tag on line 3. The dolist<\/code> loop in lines 4–7 looks at each of the other objects in the sexpr. If an object is not another sexpr, it is printed with a trailing space. If it is another sexpr, sexpr->xml<\/code> is called recursively on line 7 to process it. When all the objects in the sexpr have been processed, the end tag is printed on line 8. <\/p>\n
 The output of sexpr->xml<\/code> is a single line of XML with no formatting at all. Also, any nil<\/code>s will appear explicitly in the XML instead of the tag pair being empty and all the quoted symbols will be wrapped in <quote><\/code> … <\/quote><\/code> tags because the lisp reader turns 'symbol<\/code> into (quote symbol)<\/code>. <\/p>\n
 Now let’s look at the driver function: <\/p>\n
1:  <\/span>(defun<\/span> convert-to-xml<\/span> (sexpr)\n2:  <\/span>  (with-output-to-temp-buffer<\/span> \"*XML*\"<\/span>\n3:  <\/span>    (sexpr->xml sexpr)\n4:  <\/span>    (set-buffer \"*XML*\"<\/span>)\n5:  <\/span>    (xml-mode)\n6:  <\/span>    (replace-regexp \"\\\\bnil\\\\b<\/span>\\\\<\/span>|<\/span><quote><\/span>\\\\<\/span>|<\/span><\/quote>\"<\/span> \"\"<\/span> nil (point-min) (point-max))\n7:  <\/span>    (sgml-pretty-print (point-min) (point-max))))\n<\/pre>\n
 Lines 2 and 3 call sexpr->xml<\/code> and arrange for its output to go into a buffer named *XML*<\/code>. When sexpr->xml<\/code> returns, the *XML*<\/code> buffer is selected and set to xml-mode<\/code> (really nxml-mode<\/code>). The replace-regexp<\/code> on line 6 deletes any occurrences of nil<\/code> and gets rid of <quote><\/code> and <\/quote><\/code> tags. Finally, sgml-pretty-print<\/code> is called to format the XML nicely. <\/p>\n
 That’s a lot of work for not very much code. We could, of course, take care of the formatting and fixing up the nil<\/code> and <quote><\/code> problems right in sexpr->xml<\/code> but I wanted to show how simple the conversion can be without a lot of busy details. Besides, we have the power of Emacs so it would foolish not to use it. <\/p>\n
 The final result of running convert-to-xml<\/code> on our sample sexpr is <\/p>\n
<record<\/span>>\n  <date<\/span>>2005-02-21T18:57:39\n  <\/date<\/span>>\n  <millis<\/span>>1109041059800\n  <\/millis<\/span>>\n  <sequence<\/span>>1\n  <\/sequence<\/span>>\n  <logger<\/span>>\n  <\/logger<\/span>>\n  <level<\/span>>SEVERE\n  <\/level<\/span>>\n  <class<\/span>>java.util.logging.LogManager$RootLogger\n  <\/class<\/span>>\n  <method<\/span>>log\n  <\/method<\/span>>\n  <thread<\/span>>10\n  <\/thread<\/span>>\n  <message<\/span>>A very very bad thing has happened!\n  <\/message<\/span>>\n  <exception<\/span>>\n    <message<\/span>>java.lang.Exception\n    <\/message<\/span>>\n    <frame<\/span>>\n      <class<\/span>>logtest\n      <\/class<\/span>>\n      <method<\/span>>main\n      <\/method<\/span>>\n      <line<\/span>>30\n      <\/line<\/span>>\n    <\/frame<\/span>>\n  <\/exception<\/span>>\n<\/record<\/span>>\n<\/pre>\n
 Any Elisp coders out there might want to consider what a translator in the other direction (XML to s-expressions) would look like. Not having s-expressions to leverage makes the processing a little more difficult but not by much. <\/p>\n","protected":false},"excerpt":{"rendered":"
My last two posts mentioned the ease with which s-expressions can be converted to XML and vice versa. Out of curiosity, I decided to write an sexpr to XML function in Elisp. The implementation consists of two functions. The first, … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[9],"tags":[18,28],"class_list":["post-387","post","type-post","status-publish","format-standard","hentry","category-programming","tag-emacs","tag-emacs-lisp"],"_links":{"self":[{"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=\/wp\/v2\/posts\/387","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=387"}],"version-history":[{"count":1,"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=\/wp\/v2\/posts\/387\/revisions"}],"predecessor-version":[{"id":388,"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=\/wp\/v2\/posts\/387\/revisions\/388"}],"wp:attachment":[{"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=387"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=387"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/irreal.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=387"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}