I studied define-easy-handler macro from the hunchentoot package (which creates a function with the name NAME), and got the defun part to work, but I can't get this macro to push NAME to a list called *observers*:
(defmacro add-observer (name params &body body)
;; add NAME to the list *observers*
`(push ,name *observers*)
;; define a lisp function with the name NAME
;; with key arguments given in PARAMS
`(defun ,name (&key ,#(loop for p in params
collect p))
,#body)))
An example call to #'add-observer is:
(add-observer below-80 (id blood-sugar)
(when (< blood-sugar 80)
(format t "Patient No: ~A is hypoglycemic." id))
The function NAME is defined and works fine, but the NAME is not added to the list *observers*. It does not matter if I put both s-expressions inside a progn or not. The macroexpand clearly shows the absence of the call to push with and without a progn. What am I interpreting wrong?
EDIT
When I try this with a progn like:
`(progn
(push ...
(defn ...
it fails with Unbound variable: below-80. And when I put the backquotes back to the #'push and #'defun, again #'push doesn't work.
The macro gets the source form and computes the first result form: (push foo *observers*) and then this form is returned into the nirvana of the garbage collector, since it is not stored anywhere, not returned to any caller, not executed, ... It's garbage, immediately. So a clever compiler might even remove it...
The macro form then computes the second form (defun ...). This form is returned from the macro and then later executed.
Either you want to execute the first form at macro expansion time, then you would need to make it executable - by removing the backquote and comma.
Or you want to include it into the generated source code, then you need to return a progn form, which encloses all the sub-forms.
You also might want to think about the effects of PUSH vs. PUSHNEW, when a macro form gets executed / expanded multiple times...
Unwanted evaluation of symbols when using macros
Remember: symbols are variables in Lisp code. If you want to treat them as symbols as themselves, then you need to quote the symbol or the data structure, where they occur.
Say, your form is:
(add-observer below-80 ...)
That meansbelow-80 is unquoted.
Now you generate code where the symbol is unquoted, too.:
(push below-80 ...)
Naturally, this tries to evaluate the variable below-80, which seems to be unbound in your code.
If you want below-80 treated to be a symbol during evaluation, you have to quote it: 'below-80. Your generated code should look like this:
(push 'below-80 ...)
Quote it either in your code
(add-observer 'below-80 ...)
or by the macro into the expansion:
(push 'below-80 ...)
The backquote template for that is
`(progn
(push ',name ...)
...)
Related
Short version:
I want to change the #+ and #- reader macros to apply to all immediately subsequent tokens starting with ##, in addition to the following token. Therefore, the following code...
#+somefeature
##someattribute1
##someattribute2
(defun ...)
...would, in the absence of somefeature, result in no code.
Long version:
I have written my own readtable-macros which apply transformations to subsequent code. For example:
##traced
(defun ...)
This yields a function that writes its arguments and return values to a file, for debugging.
This fails, however, when used in conjunction with the #+ reader macro:
#+somefeature
##traced
(defun ...)
In the absence of somefeature, the function continues to be defined, albeit without the ##traced modification. This is obviously not the desired outcome.
One possible solution would be to use progn, as follows:
#+somefeature
(progn
##traced
(defun ...))
But that's kind of ugly.
I would like to modify the #+ and #- reader macros, such that they may consume more than one token. Something like this:
(defun conditional-syntax-reader (stream subchar arg)
; If the conditional fails, consume subsequent tokens while they
; start with ##, then consume the next token.
)
(setf *readtable* (copy-readtable))
(set-dispatch-macro-character #\# #\+ #'conditional-syntax-reader)
(set-dispatch-macro-character #\# #\- #'conditional-syntax-reader)
The problem is, I don't know how to "delegate" to the original reader macros; and I don't understand enough about how they were implemented to re-implement them myself in their entirety.
A naive approach would be:
(defun consume-tokens-recursively (stream)
(let ((token (read stream t nil t)))
(when (string= "##" (subseq (symbol-string token) 0 2))
(consume-tokens-recursively stream)))) ; recurse
(defun conditional-syntax-reader (stream subchar arg)
(unless (member (read stream t nil t) *features*)
(consume-tokens-recursively stream)))
However, I'm given to believe that this wouldn't be sufficient:
The #+ syntax operates by first reading the feature specification and then skipping over the form if the feature is false. This skipping of a form is a bit tricky because of the possibility of user-defined macro characters and side effects caused by the #. and #, constructions. It is accomplished by binding the variable read-suppress to a non-nil value and then calling the read function.
This seems to imply that I can just let ((*read-suppress* t)) when using read to solve the issue. Is that right?
EDIT 1
Upon further analysis, it seems the problem is caused by not knowing how many tokens to consume. Consider the following attributes:
##export expects one argument: the (defun ...) to export.
##traced expects two arguments: the debug level and the (defun ...) to trace.
Example:
#+somefeature
##export
##traced 3
(defun ...)
It turns out that #+ and #- are capable of suppressing all these tokens; but there is a huge problem!
When under a suppressing #+ or #-, (read) returns NIL!
Example:
(defun annotation-syntax-reader (stream subchar arg)
(case (read stream t nil t)
('export
(let ((defun-form (read stream t nil t)))))
; do something
('traced
(let* ((debug-level (read stream t nil t))
(defun-form (read stream t nil t)))))))
; do something
(setf *readtable* (copy-readtable))
(set-dispatch-macro-character #\# #\# #'annotation-syntax-reader)
#+(or) ##traced 3 (defun ...)
The ##traced token is being suppressed by the #+. In this situation, all the (read) calls in (annotation-syntax-reader) consume real tokens but return NIL!
Therefore, the traced token is consumed, but the case fails. No additional tokens are thus consumed; and control leaves the scope of the #+.
The (defun ...) clause is executed as normal, and the function comes into being. Clearly not the desired outcome.
The standard readtable
Changing the macros for #+ and #- is a bit excessive solution I think, but in any case remember to not actually change the standard readtable (as you did, but its important to repeat in the answer)
The consequences are undefined if an attempt is made to modify the standard readtable. To achieve the effect of altering or extending standard syntax, a copy of the standard readtable can be created; see the function copy-readtable.
§2.1.1.2 The Standard Readtable
Now, maybe I'm missing something (please give us a hint about how your reader macro is defined if so), but I think it is possible to avoid that and write your custom macros in a way that works for your use case.
Reader macro
Let's define a simple macro as follows:
CL-USER> (defun my-reader (stream char)
(declare (ignore char))
(let ((name (read stream)
(form (read stream))
(unless *read-suppress*
`(with-decoration ,name ,form)))
MY-READER
[NB: This was edited to take into account *read-suppress*: the code always read two forms, but returns nil in case it is being ignored. In the comments you say that you may need to read an indefinite number of forms based on the name of the decoration, but with *read-suppress* the recursive calls to read return nil for symbols, so you don't know which decoration is being applied. In that case it might be better to wrap some arguments in a literal list, or parse the stream manually (read-char, etc.). Also, since you are using a dispatching macro, maybe you can add a numerical argument if you want the decoration to be applied to more than one form (#2#inline), but that could be a bad idea when later the decorated code is being modified.]
Here the reader does a minimal job, namely build a form that is intended to be macroexpanded later. I don't even need to define with-decoration for now, as I'm interested in the read step. The intent is to read the next token (presumably a symbol that indicates what decoration is being applied, and a form to decorate).
I'm binding this macro to a unused character:
CL-USER> (set-macro-character #\§ 'my-reader)
T
Here when I test the macro it wraps the following form:
CL-USER> (read-from-string "§test (defun)")
(WITH-DECORATION TEST (DEFUN))
13 (4 bits, #xD, #o15, #b1101)
And here it works with a preceding QUOTE too, the apostrophe reader grabs the next form, which recursively reads two forms:
CL-USER> '§test (defun)
(WITH-DECORATION TEST (DEFUN))
Likewise, a conditional reader macro will ignore all the next lines:
CL-USER> #+(or) t
; No values
CL-USER> #+(or) §test (defun)
; No values
CL-USER> #+(or) §one §two §three (defun)
; No values
Decoration macro
If you use this syntax, you'll have nested decorated forms:
CL-USER> '§one §two (defun test ())
(WITH-DECORATION ONE (WITH-DECORATION TWO (DEFUN TEST ())))
With respect to defun in toplevel positions, you can arrange for your macros to unwrap the nesting (not completely tested, there might be bugs):
(defun unwrap-decorations (form stack)
(etypecase form
(cons (destructuring-bind (head . tail) form
(case head
(with-decoration (destructuring-bind (token form) tail
(unwrap-decorations form (cons token stack))))
(t `(with-decorations ,(reverse stack) ,form)))))))
CL-USER> (unwrap-decorations ** nil)
(WITH-DECORATIONS (ONE TWO) (DEFUN TEST ()))
And in turn, with-decorations might know about DEFUN forms and how to annotate them as necessary.
For the moment, our original macro is only the following (it needs more error checking):
(defmacro with-decoration (&whole whole &rest args)
(unwrap-decorations whole nil))
For the sake of our example, let's define a generic annotation mechanism:
CL-USER> (defgeneric expand-decoration (type name rest))
#<STANDARD-GENERIC-FUNCTION COMMON-LISP-USER::EXPAND-DECORATION (0)>
It is used in with-decorations to dispatch on an appropriate expander for each decoration. Keep in mind that all the efforts here are to keep defun in a top-level positions (under a progn), a recursive annotation would let evaluation happens (in the case of defun, it would result in the name of the function being defined), and the annotation could be done on the result.
The main macro is then here, with a kind of fold (reduce) mechanism where the forms are decorated using the resulting expansion so far. This allows for expanders to place code before or after the main form (or do other fancy things):
(defmacro with-decorations ((&rest decorations) form)
(etypecase form
(cons (destructuring-bind (head . tail) form
(ecase head
(defun (destructuring-bind (name args . body) tail
`(progn
,#(loop
for b = `((defun ,name ,args ,#body)) then forms
for d in decorations
for forms = (expand-decoration d name b)
finally (return forms))))))))))
(nb. here above we only care about defun but the loop should probably be done outside of the dispatching thing, along with a way to indicate to expander methods that a function is being expanded; well, it could be better)
Say, for example, you want to declare a function as inline, then the declaration must happen before (so that the compiler can know the source code must be kept):
(defmethod expand-decoration ((_ (eql 'inline)) name rest)
`((declaim (inline ,name)) ,#rest))
Likewise, if you want to export the name of the function being defined, you can export it after the function is defined (order is not really important here):
(defmethod expand-decoration ((_ (eql 'export)) name rest)
`(,#rest (export ',name)))
The resulting code allows you to have a single (progn ...) form with a defun in toplevel position:
CL-USER> (macroexpand '§inline §export (defun my-test-fn () "hello"))
(PROGN
(DECLAIM (INLINE MY-TEST-FN))
(DEFUN MY-TEST-FN () "hello")
(EXPORT 'MY-TEST-FN))
I'm a Lisp beginner and I'm struggling to understand why the following code gives me an error.
(dolist (elem '(mapcar
mapcon))
(when (fboundp `',elem) (print "hello")))
Thanks.
Edit:
A bit more context. I wrote the following in Elisp and I don't know how to fix it.
(dolist (ui-elem '(menu-bar-mode
tool-bar-mode
tooltip-mode
scroll-bar-mode
horizontal-scroll-bar-mode))
(when (fboundp `',ui-elem) (ui-elem -1)))
Note
In your question you mix common-lisp and elisp, but they are two different languages. The question however touches on concepts that are identical in both languages.
The need to quote symbols
The code you want to write checks if a symbol is bound to a function.
What you already know probably is that you can call fboundp on a symbol to determines this:
(fboundp 'menu-bar-mode)
=> t
When you evalute the above form, 'menu-bar-mode is the same as (quote menu-bar-mode), and is evaluated as the symbol object menu-bar-mode. This is the value that is given as an argument to fboundp.
In you example you want to iterate over a list of symbols, call fboundp on it and call the function if the symbol denotes a function. You can do this as follows:
(dolist (s '(menu-bar-mode and other symbols))
(when (fboundp s)
(funcall s -1)))
The list of symbols '(menu-bar-mode and other symbols) is quoted, which means that when dolist evaluates it, it sees a list of symbols. The value to which s is bound at each iteration of the loop is a symbol object, there is no need to quote them.
Quoting a symbol is something you have to do when writing them in your code so that they are not interpreted as variables. When you iterate over a list of symbols, you already manipulate symbols.
Note also that both Common Lisp and Emacs Lisp are "Lisp-2", meanings that you have to use (funcall ui-elem -1) instead of writing (ui-elem -1). When you write the latter form, that means calling the function literally named ui-elem because for function application, the first symbol in the list is not evaluated, it is taken literally.
Too many levels of quoting
The actual error I have when I execute your code is:
(wrong-type-argument symbolp 'mapcar)
It may look like 'mapcar denotes a symbol, because when you want the interpreter to evaluate some code as a symbol, you need to quote it. However, Lisp printers write objects in a way that they can be read back to "similar" objects. The error message that is printed if I expect a symbol to be a number is the following, where symbol foo is printed unquoted:
(+ 'foo 3)
;; error: (wrong-type-argument number-or-marker-p foo)
In your error message, the form that you are trying to use as a symbol is (quote mapcar). Recall that when you directly call fboundp:
(fboundp 'mapcar)
It is the same as-if you wrote:
(fboundp (quote mapcar))
First, (quote mapcar) is evaluated, as the symbol mapcar. Then, fboundp is applied to that value.
But when you write the following, while ui-elem is bound to symbol mapcar:
(fboundp `',ui-elem)
This is equivalent to:
(fboundp `(quote ,ui-elem))
The argument to fboundp is evaluated as (quote mapcar). You have one extra level of quoting. You could write instead:
(fboundp `,ui-elem)
But then, you don't need to use backquote/comma, you can directly write:
(fboundp ui-elem)
The problem with the following code is multiple evaluation:
(defmacro with-object (name &body body)
`(let ((,name (create-object)))
,#body
(free-object ,name)))
but I don't know how to do this the right way.
In order to use name in the let binding in your code, it has to hold a symbol. There should be no multiple evaluation problem in the code you supplied, since the symbol held by name is just given a binding in the expansion and evaluated for the free-object call within the context of that binding.
I've written an ad hoc parser generator that creates code to convert an old and little known 7-bit character set into unicode. The call to the parser generator expands into a bunch of defuns enclosed in a progn, which then get compiled. I only want to expose one of the generated defuns--the top-level one--to the rest of the system; all the others are internal to the parser and only get called from within the dynamic scope of the top-level one. Therefore, the other defuns generated have uninterned names (created with gensym). This strategy works fine with SBCL, but I recently tested it for the first time with CLISP, and I get errors like:
*** - FUNCALL: undefined function #:G16985
It seems that CLISP can't handle functions with uninterned names. (Interestingly enough, the system compiled without a problem.) EDIT: It seems that it can handle functions with uninterned names in most cases. See the answer by Rörd below.
My questions is: Is this a problem with CLISP, or is it a limitation of Common Lisp that certain implementations (e.g. SBCL) happen to overcome?
EDIT:
For example, the macro expansion of the top-level generated function (called parse) has an expression like this:
(PRINC (#:G75735 #:G75731 #:G75733 #:G75734) #:G75732)
Evaluating this expression (by calling parse) causes an error like the one above, even though the function is definitely defined within the very same macro expansion:
(DEFUN #:G75735 (#:G75742 #:G75743 #:G75744) (DECLARE (OPTIMIZE (DEBUG 2)))
(DECLARE (LEXER #:G75742) (CONS #:G75743 #:G75744))
(MULTIPLE-VALUE-BIND (#:G75745 #:G75746) (POP-TOKEN #:G75742)
...
The two instances of #:G75735 are definitely the same symbol--not two different symbols with the same name. As I said, this works with SBCL, but not with CLISP.
EDIT:
SO user Joshua Taylor has pointed out that this is due to a long standing CLISP bug.
You don't show one of the lines that give you the error, so I can only guess, but the only thing that could cause this problem as far as I can see is that you are referring to the name of the symbol instead of the symbol itself when trying to call it.
If you were referring to the symbol itself, all your lisp implementation would have to do is lookup that symbol's symbol-function. Whether it's interned or not couldn't possibly matter.
May I ask why you haven't considered another way to hide the functions, i.e. a labels statement or defining the functions within a new package that exports only the one external function?
EDIT: The following example is copied literally from an interaction with the CLISP prompt.
As you can see, calling the function named by a gensym is working as expected.
[1]> (defmacro test ()
(let ((name (gensym)))
`(progn
(defun ,name () (format t "Hello!"))
(,name))))
TEST
[2]> (test)
Hello!
NIL
Maybe your code that's trying to call the function gets evaluated before the defun? If there's any code in the macro expansion besides the various defuns, it may be implementation-dependent what gets evaluated first, and so the behaviour of SBCL and CLISP may differ without any of them violating the standard.
EDIT 2: Some further investigation shows that CLISP's behaviour varies depending upon whether the code is interpreted directly or whether it's first compiled and then interpreted. You can see the difference by either directly loading a Lisp file in CLISP or by first calling compile-file on it and then loading the FASL.
You can see what's going on by looking at the first restart that CLISP offers. It says something like "Input a value to be used instead of (FDEFINITION '#:G3219)." So for compiled code, CLISP quotes the symbol and refers to it by name.
It seems though that this behaviour is standard-conforming. The following definition can be found in the HyperSpec:
function designator n. a designator for a function; that is, an object that denotes a function and that is one of: a symbol (denoting the function named by that symbol in the global environment), or a function (denoting itself). The consequences are undefined if a symbol is used as a function designator but it does not have a global definition as a function, or it has a global definition as a macro or a special form. See also extended function designator.
I think an uninterned symbol matches the "a symbol is used as a function designator but it does not have a global definition as a function" case for unspecified consequences.
EDIT 3: (I can agree that I'm not sure whether CLISP's behaviour is a bug or not. Someone more experienced with details of the standard's terminology should judge this. It comes down to whether the function cell of an uninterned symbol - i.e. a symbol that cannot be referred to by name, only by having a direct hold on the symbol object - would be considered a "global definition" or not)
Anyway, here's an example solution that solves the problem in CLISP by interning the symbols in a throwaway package, avoiding the matter of uninterned symbols:
(defmacro test ()
(let* ((pkg (make-package (gensym)))
(name (intern (symbol-name (gensym)) pkg)))
`(progn
(defun ,name () (format t "Hello!"))
(,name))))
(test)
EDIT 4: As Joshua Taylor notes in a comment to the question, this seems to be a case of the (10 year old) CLISP bug #180.
I've tested both workarounds suggested in that bug report and found that replacing the progn with locally actually doesn't help, but replacing it with let () does.
You can most certainly define functions whose names are uninterned symbols. For instance:
CL-USER> (defun #:foo (x)
(list x))
#:FOO
CL-USER> (defparameter *name-of-function* *)
*NAME-OF-FUNCTION*
CL-USER> *name-of-function*
#:FOO
CL-USER> (funcall *name-of-function* 3)
(3)
However, the sharpsign colon syntax introduces a new symbol each time such a form is read read:
#: introduces an uninterned symbol whose name is symbol-name. Every time this syntax is encountered, a distinct uninterned symbol is created. The symbol-name must have the syntax of a symbol with no package prefix.
This means that even though something like
CL-USER> (list '#:foo '#:foo)
;=> (#:FOO #:FOO)
shows the same printed representation, you actually have two different symbols, as the following demonstrates:
CL-USER> (eq '#:foo '#:foo)
NIL
This means that if you try to call such a function by typing #: and then the name of the symbol naming the function, you're going to have trouble:
CL-USER> (#:foo 3)
; undefined function #:foo error
So, while you can call the function using something like the first example I gave, you can't do this last one. This can be kind of confusing, because the printed representation makes it look like this is what's happening. For instance, you could write such a factorial function like this:
(defun #1=#:fact (n &optional (acc 1))
(if (zerop n) acc
(#1# (1- n) (* acc n))))
using the special reader notation #1=#:fact and #1# to later refer to the same symbol. However, look what happens when you print that same form:
CL-USER> (pprint '(defun #1=#:fact (n &optional (acc 1))
(if (zerop n) acc
(#1# (1- n) (* acc n)))))
(DEFUN #:FACT (N &OPTIONAL (ACC 1))
(IF (ZEROP N)
ACC
(#:FACT (1- N) (* ACC N))))
If you take that printed output, and try to copy and paste it as a definition, the reader creates two symbols named "FACT" when it comes to the two occurrences of #:FACT, and the function won't work (and you might even get undefined function warnings):
CL-USER> (DEFUN #:FACT (N &OPTIONAL (ACC 1))
(IF (ZEROP N)
ACC
(#:FACT (1- N) (* ACC N))))
; in: DEFUN #:FACT
; (#:FACT (1- N) (* ACC N))
;
; caught STYLE-WARNING:
; undefined function: #:FACT
;
; compilation unit finished
; Undefined function:
; #:FACT
; caught 1 STYLE-WARNING condition
I hope I get the issue right. For me it works in CLISP.
I tried it like this: using a macro for creating a function with a GENSYM-ed name.
(defmacro test ()
(let ((name (gensym)))
`(progn
(defun ,name (x) (* x x))
',name)))
Now I can get the name (setf x (test)) and call it (funcall x 2).
Yes, it is perfectly fine defining functions that have names that are unintenred symbols. The problem is that you cannot then call them "by name", since you can't fetch the uninterned symbol by name (that is what "uninterned" means, essentially).
You would need to store the uninterned symbol in some sort of data structure, to then be able to fetch the symbol. Alternatively, store the defined function in some sort of data structure.
Surprisingly, CLISP bug 180 isn't actually an ANSI CL conformance bug. Not only that, but evidently, ANSI Common Lisp is itself so broken in this regard that even the progn based workaround is a courtesy of the implementation.
Common Lisp is a language intended for compilation, and compilation produces issues regarding the identity of objects which are placed into compiled files and later loaded ("externalized" objects). ANSI Common Lisp requires that literal objects reproduced from compiled files are only similar to the original objects. (CLHS 3.2.4 Literal Objects in Compiled Files).
Firstly, according to the definition similarity (3.2.4.2.2 Definition of Similarity), the rules for uninterned symbols is that similarity is name based. If we compile code with a literal that contains an uninterned symbol, then when we load the compiled file, we get a symbol which is similar and not (necessarily) the same object: a symbol which has the same name.
What if the same uninterned symbol is inserted into two different top-level forms which are then compiled as a file? When the file is loaded, are those two similar to each other at least? No, there is no such requirement.
But it gets worse: there is also no requirement that two occurrences of the same uninterned symbol in the same form will be externalized in such a way that their relative identity is preserved: that the re-loaded version of that object will have the same symbol object in all the places where the original was. In fact, the definition of similarity contains no provision for preserving the circular structure and substructure sharing. If we have a literal like '#1=(a b . #1#), as a literal in a compiled file, there appears to be no requirement that this be reproduced as a circular object with the same graph structure as the original (a graph isomorphism). The similarity rule for conses is given as naive recursion: two conses are similar if their respective cars and cdrs are similar. (The rule can't even be evaluated for circular objects; it doesn't terminate).
That the above works is because of implementations going beyond what is required in the spec; they are providing an extension consistent with (3.2.4.3 Extensions to Similarity Rules).
Thus, purely according to ANSI CL, we cannot expect to use macros with gensyms in compiled files, at least in some ways. The expectation expressed in code like the following runs afoul of the spec:
(defmacro foo (arg)
(let ((g (gensym))
(literal '(blah ,g ,g ,arg)))
...))
(defun bar ()
(foo 42))
The bar function contains a literal with two insertions of a gensym, which according to the similarity rules for conses and symbols need not reproduce as a list containing two occurrences of the same object in the second and third positions.
If the above works as expected, it's due to "extensions to the similarity rules".
So the answer to the "Why can't CLISP ..." question is that although CLISP does provide an extension for similarity which preserves the graph structure of literal forms, it doesn't do it across the entire compiled file, only within individual top level items within that file. (It uses *print-circle* to emit the individual items.) The bug is that CLISP doesn't conform to the best possible behavior users can imagine, or at least to a better behavior exhibited by other implementations.
I have a program that takes as inputs a chunk of data and a list of rules, applying both a set of standard rules and the rules given as input to the chunk of data. The size of both inputs may vary.
I want to be able to write a list of rules like this:
(rule-generating-macro
(rule-1-name rule-1-target
(rule-action-macro (progn actions more-actions)))
(rule-2-name rule-2-target
(rule-action-macro (or (action-2) (default-action))))
;; more rules
)
Right now, rules are more verbose -- they look more like
(defvar rule-list
`((rule-1-name rule-1-target
,#(rule-action-macro (progn actions more-actions)))
(rule-2-name rule-2-target
,#(rule-action-macro (or (action-2) (default-action))))
;; more rules
)
The latter form looks uglier to me, but I can't figure out how to write a macro that can handle a variable-length &rest argument, iterate over it, and return the transformed structure. Using a defun instead of a defmacro isn't really on the table because (as hopefully the example shows) I'm trying to control evaluation of the list of rules instead of evaluating the list when my program first sees it, and once you need to control evaluation, you're in defmacro territory. In this case, the thorny point is the rule-action-macro part - getting the interpreter to read that and use its expanded value has been problematic.
How can I create a macro that handles a variable-length argument so that I can write rule lists in a concise way?
defmacro will happily accept a &rest argument
(see Defining Macros for Emacs Lisp and Macro Lambda Lists for Common Lisp).
Then you can do pretty much anything you want with it in the macro body - e.g., iterate over it. Remember, macro is much more than just backquote!
E.g.:
(defmacro multidefvar (&rest vars)
(let ((forms (mapcar (lambda (var) `(defvar ,var)) vars)))
`(progn ,#forms)))
(macroexpand '(multidefvar a b c d))
==> (PROGN (DEFVAR A) (DEFVAR B) (DEFVAR C) (DEFVAR D))