+ A very simple xml parser

This parses something LIKE xml, not XML itself. It serves as yet another
example of how the transformator can be used to achieve different tree
walk goals.

example/simple_xml.rb

@@ -0,0 +1,60 @@
+# A simple xml parser. It is simple in the respect as that it doesn't address
+# any of the complexities of XML. This is ruby 1.9.
+
+$:.unshift '../lib'
+
+require 'pp'
+require 'parslet'
+
+module XML
+  include Parslet
+
+  root :document
+
+  rule(:document) {
+    tag(close: false).as(:o) >> document.as(:i) >> tag(close: true).as(:c) |
+    text
+  }
+
+  # Perhaps we could have some syntax sugar to make this more easy?
+  #
+  def tag(opts={})
+    close = opts[:close] || false
+
+    parslet = str('<')
+    parslet = parslet >> str('/') if close
+    parslet = parslet >> (str('>').absnt? >> match("[a-zA-Z]")).repeat(1).as(:name)
+    parslet = parslet >> str('>')
+
+    parslet
+  end
+
+  rule(:text) {
+    match('[^<>]').repeat(0)
+  }
+end
+
+def check(xml)
+  include XML
+  r=parse(xml)
+
+  # We'll validate the tree by reducing valid pairs of tags into simply the string
+  # ok. If the transformation ends on a string, then the document was 'valid'. 
+  t = Parslet::Transform.new
+  t.rule(
+    o: {name: simple(:o)}, 
+    c: {name: simple(:c)}, 
+    i: simple(:t)
+  ) { |d| 
+    if d[:o] == d[:c]
+      "ok." 
+    else
+      fail "Open tag doesn't match close tag ... #{d.inspect}"
+    end
+  }
+
+  t.apply(r)
+end
+
+pp check("<a><b>some text in the tags</b></a>")
+pp check("<b><b>some text in the tags</b></a>")