macro-function

(macro-function symbol &optional environment)

The macro expander of symbol, or nil when the name is a function, one of the 25 special operators, or unknown. It answers non-nil for a user macro defined with defmacro, for every built-in macro (the names rontolisp:list-macros reports), and for the Common Lisp macros rontolisp implements as special forms of its own (defun, handler-case, dolist, ...) -- together, every name a caller may not apply.

The environment argument is accepted and ignored: macrolet bodies are expanded away before any body runs, so the global answer is the only one there is.

On the interpreter the value is the real expander -- a single-step expansion callable as (funcall expander form environment):

A COMPILED program has no macro table left (macros are fully expanded before the backends see the program), so there the value is a stub: the predicate above is exact on all four backends, but calling it signals macro-function: a compiled program cannot expand a macro at run time.

The setf place is supported for one shape only: (setf (macro-function 'new) (macro-function 'existing)) gives an existing defmacro-defined macro a second name sharing its expander, so both names expand identically from then on. Anything else -- an arbitrary expander function, or a name that is not a user macro -- signals an error, because there is no macro function object to store.

Lite: a non-symbol argument answers nil where Common Lisp signals a type error.