Question

I’m confused about the difference between a macro and a function; specifically why PART TWO fails below given that PART ONE succeeds.

(defun foo () "foo")
(setf a 3) ;; sets the symbol value cell to 3
(setf a #'foo) ;; PART ONE
(funcall a) ;; returns foo

(defmacro bar () "bar")
(setf b #'bar) ;; Error the macro name bar was found as an argument to function PART TWO

Answer 1

The # (sharpsign) is a standard macro character which is a dispatch macro character. It’s supposed to compose with another character. The #' (sharpsign single-quote) combination expects a function name or lambda expression after it, and it expands to (function expression).

So, #'foo is expanded at read-time to (function foo). If foo is a function, function will evaluate to it. In a lexical scope, it may be a foo fbound by a flet or labels. If there’s no such lexical definition, it’ll try to fetch the global function definition from the symbol’s function.

Now, (function bar) signals an error when bar represents a macro, be it a lexical macrolet or a global defmacro. You may, however, use (macro-function 'bar) to fetch the macro function of the global bar macro. If it exists, it’s a function of two arguments: a form and an environment.

Unless you’re going to apply bar‘s macro function to forms, it’s probably not what you want. Let’s think about applying the macro function of and: it will not do a logical boolean operation, it’ll probably expand the given form into an if.

However, if this is what you want, remember that macro-function has a second optional parameter, an environment. You may get an environment as argument in a defmacro or in a define-setf-expander. Within the latter, it’s usually needed so that get-setf-expansion takes into account the lexical environment in expanding a subform.

Try this out:

(funcall (macro-function 'and) '(and form1 form2 form3) nil)

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Exercise: Implement your own macroexpand-1 and macroexpand.

Exercise: Implement a macroexpand-all, a macroexpand that recurses into subforms, recognizing Common Lisp’s special operators.

Note: Don’t get too far with macroexpand-all, it needs a code walker, which is implementation-specific.