Symbol macros

A symbol macro is a symbol that macroexpands into a lisp form. It is similar to a preprocessor macro in C, but it must expand into a syntactically complete expression. Symbol macros are the underlying technology behind the with-slots and with-accessors macros. They allow you to introduce an identifier that appears to be a lexical variable, but actually executes some arbitrary code to compute a value. So we can place the storage for a variable in a vector or in an instance, and use a symbol macro to make it appear to be an ordinary variable.

Gerry Sussman doesn't like symbol macros. They are a lie. It appears that you are just doing an ordinary variable access, which should be a quick and simple operation, but in fact you could be executing arbitrary code. This can lead to some nasty suprises.

But in my opinion, you shouldn't discard a useful tool just because there is a way to misuse it. If your symbol macro is just redirecting a variable to a slot in an instance, there is little harm in that.