zz note: traversing dynamic type spaces

Looking at zz note: important dynamic programming patterns, one important pattern is traversing dynamic type spaces.

By this we mean, allow traversal of foreign parsed data using simple native-looking syntax.

<customer name="David" Phone="555.1212"/>
<customer name="Fred" Phone="555"1313"/>
<customer name="Suzy" Phone="555.1414>
    <order id=5223 date=... />
  </customer>

Using something like the Python meta-object protocol, accessing data in this tree can be as simple as:

  for customer in dom._all("customer"):
     print "Name: " + customer.name
     for orders in customer._all("order"):
          print "   Order:" + orders.id

However, in a typical static typed DOM API, this typically turns into a morass of quoted strings and function calls. For example, using the C# XmlDocument, one way to write this code is below. Note that we are not getting any static safety over the DOM contents using this larger code. It's just more confusing and more typing.

foreach(var node in dom.GetElementsByTagName("customer")) {
   System.println("Name: " .. 
      node.Attributes.ToDictionary(p => p.Name.toLower())["name"].Value);
   foreach(var orders in node.GetElementsByTagName("order")) {
      System.println("    Order:" .. 
      orders.Attributes.ToDictionary(p => p.Name.toLower())["id"].Value);
   }
}

C# / CLR also has a type of dynamic object with a Python-like runtime meta-object protocol. Using these objects, we can express this DOM traversal in the same way Python can:

foreach(dynamic customer in dom._all("customer")) {
  System.println("Name: " .. customer.name);
  foreach(dynamic order in customer._all("order")) {
     System.println("    Order: " .. order.id);
  }
}