From: Phrogz <gavin@...>
Date: 2006-09-01T08:40:46+09:00
Subject: Fun with XEXPR

Just sharing some fun code. I think I cranked this out in 30 minutes.
Ruby is so cool.
It's not a full specification of the XEXPR language
http://www.w3.org/TR/xexpr/, but at this point that's just a trivial
amount of additional work.

require 'rexml/document'
include REXML

class Element
  def self.handle_expression( expr_name, &eval_code )
    (@@expression_code ||= {})[ expr_name ] = eval_code
  end

  def run_expression
    unless proc = @@expression_code[ self.name ]
      raise "I don't know how to handle '#{self.name}' expressions yet"
    end
    proc.call( self )
  end

  def run_children
    last_value = nil
    self.elements.each{ |c|
      last_value = c.run_expression
    }
    last_value
  end
end

Element.handle_expression( 'expr' ){ |me|
  me.run_children
}

Element.handle_expression( 'set' ){ |me|
  name = me.attributes[ 'name' ]
  val =  me.attributes[ 'value' ] || me.run_children
  (@@global_expression_variables ||= {})[ name ] = val
}

Element.handle_expression( 'get' ){ |me|
  name = me.attributes[ 'name' ] || me.run_children
  (@@global_expression_variables ||= {})[ name ]
}

Element.handle_expression( 'print' ){ |me|
  puts name = me.attributes[ 'string' ] || me.run_children
  name
}

Element.handle_expression( 'if' ){ |me|
  if me.elements[ 1 ].run_expression
    me.elements[ 2 ].run_expression
  else
    me.elements[ 3 ] && me.elements[ 3 ].run_expression
  end
}

Element.handle_expression( 'false' ){ false }

Element.handle_expression( 'add' ){ |me|
  case me.elements[ 1 ].run_expression
    when String
      me.elements.to_a.map{ |el| el.run_expression }.join
    else
      me.elements.to_a.inject(0){ |sum,el| sum + el.run_expression }
  end
}

Element.handle_expression( 'string' ){ |me|
  me.text.to_s
}

d = REXML::Document.new( <<ENDXML )
<expr>
  <set name="location" value="World" />
  <print>
    <if>
      <false />
      <string>BAD NEWS!</string>
      <add>
        <string>Hello </string>
        <get>
          <string>location</string>
        </get>
      </add>
    </if>
  </print>
</expr>
ENDXML

d.root.run_expression
#=> Hello World