SXML
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| SXML | |
|---|---|
| Filename extension |
.sxml, .scm |
| Type code | TEXT |
| Type of format | markup language |
SXML is an alternative syntax for writing XML data, using the form of S-expressions. It is also a set of implementations that provide typical XML-processing functionalities that operate on the SXML syntax.
Textual correspondence between SXML and XML for a sample XML snippet is shown below:
| XML | SXML |
|---|---|
<tag attr1="value1"
attr2="value2">
<nested>Text node</nested>
<empty/>
</tag>
|
(tag (@ (attr1 "value1")
(attr2 "value2"))
(nested "Text node")
(empty))
|
The following two observation can be drawn from the above example:
- Textual notations for XML and SXML are much alike; informally, SXML textually differs from XML in relying on round brackets instead of angular braces.
- Additionally, SXML is not only a straightforward textual notation for XML data, but also has a directly-corresponding primary data structure for the LISP family of functional programming languages, thus providing an illustrative approach for processing XML data with a general-purpose programming language.
Similarity between XML and S-expressions reified in SXML allows achieving close integration between XML data and programming language expressions, resulting in illustrativeness and simplicity of XML data processing for an application programmer.
The structural similarity of S-expression-like and XML-like syntaxes has often been discussed in the XML community, going at least as far back as 1993.[1][2][3][4]
Example
Take the following simple XHTML page:
<html xmlns="http://www.w3.org/1999/xhtml"
xml:lang="en" lang="en">
<head>
<title>An example page</title>
</head>
<body>
<h1 id="greeting">Hi, there!</h1>
<p>This is just an >>example<< to show XHTML & SXML.</p>
</body>
</html>
After translating it to SXML, the same page now looks like this:
(*TOP* (@ (*NAMESPACES* (x "http://www.w3.org/1999/xhtml")))
(x:html (@ (xml:lang "en") (lang "en"))
(x:head
(x:title "An example page"))
(x:body
(x:h1 (@ (id "greeting")) "Hi, there")
(x:p "This is just an >>example<< to show XHTML & SXML."))))
Each element's tag pair is replaced by a set of parentheses. The tag's name is not repeated at the end, it is simply the first symbol in the list. The element's contents follow, which are either elements themselves or strings. There is no special syntax required for XML attributes. In SXML they are simply represented as just another node, which has the special name of @. This can't cause a name clash with an actual "@" tag, because @ is not allowed as a tag name in XML. This is a common pattern in SXML: anytime a tag is used to indicate a special status or something that is not possible in XML, a name is used that does not constitute a valid XML identifier.
We can also see that there's no need to "escape" otherwise meaningful characters like & and > as & and > entities. All string content is automatically escaped because it is considered to be pure content, and has no tags or entities in it. This also means it is much easier to insert autogenerated content and that there is no danger that we might forget to escape user input when we display it to other users (which could lead to all kinds of nasty cross-site scripting attacks or other annoyances).
SXML shortcomings
Other considerations
Of course SXML can be parsed by a program in any programming language, and then be represented using any desired data structure. Precisely as with XML, implementations vary: XML applications that can process data in a one-pass serial fashion typically use SAX style interfaces that stay very close to the raw input data stream, while applications that must access parts of the data in non-linear random-access fashion use DOM interfaces that mirror the hierarchical structure instead.
It has been claimed that because the underlying structure is based on singly linked lists, nodes have no default access to either the parent node and the siblings nodes, only to their child nodes. But this confuses underlying structure, with a linear representation of a structure. Any disk file is a linear sequence of bytes or characters—but that mundane fact places almost no limits on what structures can be represented.
As a simple example, saying that the following expression's "underlying structure" is either a 21-character string, or a singly-linked list of 11 nodes (4 numbers, 3 arithmetic operators, and 4 grouping delimiters), is at best a gross oversimplification:
( 1 + 2 ) * ( 3 + 4 )
Because SXML is so similar to S-expressions syntactically, it is trivial to load it into a LISP or Scheme program just as if it were a generic S-expression. Doing so is utterly trivial to program in such languages, but would lead to each parenthesized group becoming a singly-linked list: a data structure which is far from optimal for kinds of processing commonly anticipated for XML-like structures. Similarly, in any programming language it is trivial to load an entire SXML document into one long string—but it would be a poor choice for most purposes.
In reality, XML, SXML, SGML, or most any data representation is loaded into data structures that facilitate required operations. DOM and other interfaces provide methods to get from an element to its parent, preceding and following siblings, and numbered children directly, and to access attributes by name. Practical DOM implementations make likely operations very fast.[5]
If a program does not do this, then typical operations such as getting the Nth child of an element, or the preceding element in a long list, or the element with a given ID, remain possible but are far from optimal.
Citations
- ^ Tim Berners-Lee. "Reform of SGML." March 1993. http://www.w3.org/MarkUp/SGML/TimComments
- ^ Joe English. "Delimited pseudoelements". Oct 1, 1996. http://lists.w3.org/Archives/Public/w3c-sgml-wg/1996Oct/0021.html
- ^ Ora Lassila. "PICS-NG Metadata Model and Label Syntax W3C NOTE. 1997-05-14 (section 6.2). http://www.w3.org/TR/NOTE-pics-ng-metadata.html
- ^ "CL-XML Provides Common Lisp Support for XML, XPath, and XQuery." The Cover Pages, June 9, 2001. http://xml.coverpages.org/ni2001-06-09-a.html
- ^ Steven DeRose. "Architecture and Speed of Common XML Operations." In Proceedings of Extreme Markup Languages. Montreal, 2005.
External links
- SXML Tools Tutorial by Dmitry Lizorkin
- Main SSAX/SXML page
- XML Matters: Investigating SXML and SSAX: Manipulating XML in the Scheme programming language by David Mertz, Ph.D. IBM developerWorks article
Detailed introduction, motivation and real-life case-studies of SSAX, SXML, SXPath and SXSLT. The paper and the complementary talk presented at the International Lisp Conference 2002.