Writing XML

[Tomek Sowiński]

While I'm circling the problem of parsing, I took a quick look at writing not to get stuck in analysis-paralysis. Writing XML is pretty independent from parsing and an order of magnitude easier to solve. It was perfect to get myself coding.

These are the guidelines I followed:

 * Memory minimalism: don't force allocating an intermediate node structure just to push a few tags down the wire.

 * Composability: operating on an arbitrary string output range.

 * Robustness: tags should not be left open, even if the routine producing tag interior throws.

 * Simplicity of syntax: resembling real XML if possible.

 * Space efficiency / readability: can write tightly (without indents and newlines) for faster network transfer and, having easy an means for temporary tight writing, for better readability.

 * Ease of use:
   - automatic to!string of non-string values,
   - automatic string escaping according to XML standard,
   - handle nulls: close the tags short (<tag/>), don't write attributes with null values at all.

 * anything else?


The new writer meets pretty much all of the above. Here's an example to get a feel of it:

auto books = [
    Book([Name("Grębosz", "Jerzy")], "Pasja C++", 1999),
    Book([Name("Navin", "Robert", "N.")], "Mathemetics of Derivatives", 2007),
    Book([Name("Tokarczuk", "Olga")], "Podróż ludzi Księgi", 1996),
    Book([Name("Graham", "Ronald", "L."),
         Name("Knuth", "Donald", "E."),
         Name("Patashnik", "Oren")], "Matematyka Konkretna", 2008)
];

auto outputRange = ... ;
auto xml = xmlWriter(outputRange);

xml.comment(books.length, " favorite books of mine.");
foreach (book; books) {
    xml.book("year", book.year, {
         foreach (author; book.authors) {
             xml.tight.authorName({
                 xml.first(author.first);
                 xml.middle(author.middle);
                 xml.last(author.last);
             });
         }
         xml.tight.title(book.title);
    });
}

--------------------------------- program output ---------------------------------

<!-- 4 favorite books of mine. -->
<book year="1999">
  <authorName><first>Jerzy</first><middle/><last>Grębosz</last></authorName>
  <title>Pasja C++</title>
</book>
<book year="2007">
  <authorName><first>Robert</first><middle>N.</middle><last>Navin</last></authorName>
  <title>Mathemetics of Derivatives</title>
</book>
<book year="1996">
  <authorName><first>Olga</first><middle/><last>Tokarczuk</last></authorName>
  <title>Podróż ludzi Księgi</title>
</book>
<book year="2008">
  <authorName><first>Ronald</first><middle>L.</middle><last>Graham</last></authorName>
  <authorName><first>Donald</first><middle>E.</middle><last>Knuth</last></authorName>
  <authorName><first>Oren</first><middle/><last>Patashnik</last></authorName>
  <title>Matematyka Konkretna</title>
</book>


Questions and comments?

-- 
Tomek