Some simple examples of using Erlang’s XPath implementation

原文地址 [url]http://www.lshift.net/blog/2008/01/31/some-simple-examples-of-using-erlangs-xpath-implementation[/url]

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这篇文章很好的介绍了xmerl_xpath 这个很方便的东西, 而在官方的文档里, 一笔带过, 让人不识宝,我把它挖掘出来,有兴趣的同学折腾折腾...[/color]


We’ve been investigating the possibility of an XPath-based routing extension to RabbitMQ, where XPath would be used as binding patterns, and the message structure would be exposed as XML infoset. As part of this work, we’ve been looking at Erlang’s XPath implementation that comes as part of the built-in xmerl library.

Here are a couple of examples of Erlang’s XPath in action. First, let’s parse a document to be queried:

{ParsedDocumentRootElement, _RemainingText = ""} =
xmerl_scan:string("<foo>" ++
"<myelement myattribute=\"red\">x</myelement>" ++
"<myelement myattribute=\"blue\">x</myelement>" ++
"<myelement myattribute=\"blue\">y</myelement>" ++
"</foo>").

(We could have used xmerl_scan:file to read from an external file, instead of xmerl_scan:string, if we’d wanted to.)

Next, let’s retrieve the contents of every myelement node that contains text exactly matching “x”:

69> xmerl_xpath:string("//myelement[. = 'x']/text()”,
ParsedDocumentRootElement).
[#xmlText{parents = [{myelement,1},{foo,1}],
pos = 1,
language = [],
value = “x”,
type = text},
#xmlText{parents = [{myelement,2},{foo,1}],
pos = 1,
language = [],
value = “x”,
type = text}]

Notice that it’s returned two XML text nodes, and that the “parents” elements differ, corresponding to the different paths through the source document to the matching nodes.

Next, let’s search for all myelements that have a myattribute containing the string “red”:

72> xmerl_xpath:string("//myelement[@myattribute='red']“,
ParsedDocumentRootElement).
[#xmlElement{
name = myelement,
expanded_name = myelement,
nsinfo = [],
namespace = #xmlNamespace{default = [],nodes = []},
parents = [{foo,1}],
pos = 1,
attributes =
[#xmlAttribute{
name = myattribute,
expanded_name = [],
nsinfo = [],
namespace = [],
parents = [],
pos = 1,
language = [],
value = “red”,
normalized = false}],
content =
[#xmlText{
parents = [{myelement,1},{foo,1}],
pos = 1,
language = [],
value = “x”,
type = text}],
language = [],
xmlbase = “/localhome/tonyg”,
elementdef = undeclared}]

This time, there’s only the one match. Finally, a query that no nodes satisfy:

75> xmerl_xpath:string("//myelement[@myattribute='red' and . = 'y']“,
ParsedDocumentRootElement).
[]

If we had replaced the 'y' with 'x', we’d have retrieved a non-empty nodeset.