rntz notes that this isn't as powerful as symbol macros, and that's true: this lets you specify the behavior of a global variable at run time, while symbol macros would let you manipulate the effect of a symbol during macro expansion (compile time).
I don't get symbol macros. Is there an example of a symbol macro in use? On another thread someone mentioned that a symbol macro can expand into arbitrary code. Is this the same as an ordinary macro with no arguments (except for a pair of parens) ?
On a possibly related note, when compiling (assign ...) forms, ac.scm calls macex on the 1st arg:
(assign (foo bar) 12)
'(foo bar) gets macexed. Later, ac-set requires that its first argument be a symbol. So '(foo bar) must expand into a symbol. I didn't see this obviously being used anywhere in arc ... do I need to look harder? Is its usage buried under layers of macros (deep inside '= for example), or what's the story? Is this somehow related to the concept of symbol macros?
I'm not sure about your related note, but for an example of symbol macros in use:
This example is from Common Lisp's object system, called CLOS. When you want to do some code while accessing specific slots of an object, you can use a 'with-slots form, which lets you name symbols that you'll use to reference specific slots of the object. The 'with-slots form then generates local symbol-macros for each symbol you mentioned, which expand into code to access those slots. An example, taken from PCL :
(with-slots ((bal balance)) account ; Here 'balance is the name of the slot, and 'account is the specific object
(when (< bal *minimum-balance*) ; *symbol* is a CL convention for global variables
(decf bal (* bal .01))))) ; decf is like --
 Practical Common Lisp, by Peter Seibel, available online at www.gigamonkeys.com/book/