Helma Logo
main list history
previous version  overview  next version

Version 29 by hannes on 09. May 2008, 14:53

The javascript import features in *Helma NG* try to loosely imitate the *semantics of python's import statement|http://www.effbot.org/zone/import-confusion.htm* in order to allow applications to be written in a truly modular way. The magic of python's import statement is that a script never has to do anything special in order to avoid name clashes with other scripts, because every script lives in its one top level scope by default.

To sum it up:

# You don't have to use any special Javascript syntax to denote a namespace. Namespaces are invisible when you're insideEvery script is by its nature a separate scope, and defined assigned to a namespace by the file name from the outsidescript that imported it.
# It's hardly possible virtually impossible to produce a name collision, as each script lives in its own scope.
# Since scripts live in their own scope, imported scripts are only visible to the scripts that imported them.

=== Implementation

Helma NG implements this by using a <a href="http://www.mozilla.org/rhino/scopes.html#sharingscopes">separate top-level scope</a> for each loaded script file, using a shared top-level scope as prototype. A simplistic implementation of most of the behaviour described above in the Rhino shell may look like this:

  // set reference to global object
  var __global__ = __global__ || this;
  function include(script, as) {
    var scope = new Object();
    scope.__parent__ = null;
    scope.__proto__ = __global__;
    // scope is now a top level scope that inherits
    // from the global, shared top level scope
    this[as] = scope;

Of course, Helma NG also takes care of script reloading and stuff, and the shared module scope is really a per-thread scope that uses a really-shared scope for the really-shared stuff.

=== Example

Suppose there is a Javascript file called <code>helma/database.js</code> with the following content:

  function getConnection(...) {
  function Connection(...) {
  function Query(...) {

Then consider the following statements in another script file:

<dd>This makes the functions defined in helma/database.js available in the current scope as
<dt><b>importModuleAs(<dt><b>importModule("helma.database", "db")</b></dt>
<dd>This makes the functions defined in helma/database.js available in the current scope as
<dt><b>importFromModule("helma.database", "getConnection", "Query")</b></dt>
<dd>This makes the functions defined in helma/database.js available in the current scope as

Note that the functions in helma/database.js do not care in which namespace they are loaded. They only see what is defined in their local file, and in the global scope shared by all modules. The scope that loaded the module is absolutely invisible to the loaded module! Also note that the getConnection() function in the last example can still access the Connection() constructor although it's not included in the importFromModule(). That's the power of closures!