ES4 note: Reserved namespaces

Lars Hansen lhansen at adobe.com
Thu Apr 17 13:38:50 PDT 2008


We've had this notion for a while that some namespaces are 
"reserved", that is, you can't use them for your own
properties except in well-defined ways.  The notion of
reserved namespaces seems to be important for future-proofing
the language: we may wish to introduce new names in those
namespaces and it's important that user definitions not get
in the way.

Without teeth there is no way that we can make the notion 
of reserved namespaces stick, so here's a brief writeup
about what we think those teeth might look like.  Please
look for cavities.

There are three important reserved namespaces, they have
different restrictions:

* "reflect" is used to tag reflection methods on type 
  objects.  User code may not define properties (including
  methods) in this namespace at all.

* "intrinsic" is used to tag instance methods. User code
  may override existing methods in this namespace in
  subclasses but may not otherwise define properties in
  this namespace.

* "meta" is used to tag static and instance catchall
  (get, set, has, delete, and invoke) methods.  User
  code may introduce or override these five methods
  but may not otherwise define properties in this
  namespace.

The rules are:

* reserved namespaces may not be aliased (ie they are illegal
  on the right hand side of "=" in "namespace ns1 = ns2")

* any attempt to introduce fixture properties on objects
  in a way that contradicts the restrictions described
  above shall cause a syntax error to be thrown.

(The namespaces are not reserved words, but we believe it
is always possible to detect statically whether a namespace
reference that might be used to introduce a fixture is to
one of the reserved namespaces; the first rule above is
not even necessary, it just makes code more readable
by requiring the reserved namespaces always to be spelled
the same way.)

The most important consequence of the rule is that when a
future revision of the language introduces a new name in a
reserved namespace, there is a guarantee (modulo vendor
extensions, but presumably they will be taken into account
if the language is revised) that there will not be a fixture
in any user program that uses that name, and any existing
dynamic properties on the object's prototype will
be shadowed by the new fixture.

Another consequence of the rules is that user code may introduce
dynamic properties in the reserved namespaces.  This is 
considered a feature; the use case is that user code may be
able to compensate for missing intrinsic, reflect, and possibly
meta methods in one implementation by introducing the
corresponding prototype methods.

--lars



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