In GemStone/Smalltalk rather than having a single SymbolDictionary (Smalltalk) that contains all the globals, there is a list of SymbolDictionary instances each of which is examined to see if it has the key being referenced. There is no built-in support for nesting—just an Array of SymbolDictionary instances instead of a single SymbolDictionary instance.

The code:

            myTest := TestCase new.

Is equivalent to:

            myTest := (System myUserProfile symboList resolveSymbol: #’TestCase’) value new.

By convention, each SymbolDictionary instance is contained in itself as a value where the key is its “name.” For example, filing in the SUnit package results in creation of a new SymbolDictionary whose initial key is #’SUnit’ with the new SymbolDictionary instance as its value. The new SymbolDictionary instance is added as the first entry in the current SymbolList instance (so that any other globals with identical names are effectively hidden), and then the file-in proceeds adding various keys including #’TestCase’, #’TestResource’, #’TestResult’, and #’TestSuite’ (each of which has an associated value that is the named class).

If I wanted to be more particular about the lookup in the above code example, I could specify:

myTest := (SUnit at: #’TestCase’) new.

Or, to be more specific:

            myTest := ((System myUserProfile symboList at: 2) at: #’TestCase’) new.

Or, to be more specific still:

            myTest := (((AllUsers userWithId: ‘JamesFoster’) symbolList at: 2) at: #’TestCase’) new.

Importantly, there is no change in the language syntax. The “magic” happens during method compilation where one of the arguments to the compiler is the SymbolList instance that is to be searched for any needed global references. Different methods can be compiled with reference to different SymbolList instances, and still call each other. This allows for private classes, class replacement, etc. It has served reasonably well for a couple decades.

James Foster

-----Original Message-----
From: Michael van der Gulik [mailto:mikevdg@gmail.com]
Sent: Wednesday, November 29, 2006 8:52 PM
To: james@foster.net; The general-purpose Squeak developers list
Subject: Re: Squeak and Namespaces

On 11/30/06, James Foster <james@foster.net> wrote:

Michael's description is something like what GemStone/Smalltalk provides—each login is assigned an array of SymbolDictionary instances (a SymbolList), and compiling is in the context of a SymbolList. You can have multiple globals visible with the same name, but the first one found is used. Different code can be compiled with different instances of SymbolList, and multiple instances of SymbolList can reference the same SymbolDictionary. Security can be addressed by giving a user a SymbolList that does not reference things that should be hidden.

That sounds interesting.

- How do you specify that. rather than the first global found, you want to use a different global with the same name?

- Can SymbolDictionaries be nested? I.e. Collections::Sequenceable::OrderedCollection?

Michael.