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cl-indeterminism

Do some cool stuff with undefined functions and variables.

By now, 'cool stuff' includes:

  • FIND-UNDEFS - described in section "Finding"
  • MACROEXPAND-ALL-TRANSFORMING-UNDEFS - described in section "Transforming"
  • MACROEXPAND-CC-ALL-TRANSFORMING-UNDEFS - described in section "Transforming at compile-time"

Finding

Codewalk the form and find, which variables and functions are undefined.

CL-USER> (ql:quickload 'cl-indeterminism)
CL-USER> (cl-indeterminism:find-undefs '(foo bar baz))
((:FUNCTIONS FOO) (:VARIABLES BAZ BAR))

FIND-UNDEFS is now a macro which expands in the surrounding context, making it possible to catch undefined variables relative to the current lexenv.

CL-USER> (let ((a 1)) (cl-indeterminism:find-undefs 'a))
((:FUNCTIONS) (:VARIABLES))

Still, one can explicitly specify to find undefs with respect to top-level environment

CL-USER> (let ((a 1)) (cl-indeterminism:find-undefs 'a :env :null))
((:FUNCTIONS) (:VARIABLES A))

Can be also used to find list of undefined variables at compile time

(defmacro list-the-undefs (&body body &environment env)
  `(list ,@(mapcar (lambda (x) `(quote ,x))
           (find-undefs `(progn ,@body) :env env))))
(let ((bar 1)) (declare (ignore bar)) (find-undefs '(foo bar baz)))
((:FUNCTIONS FOO) (:VARIABLES BAZ))

Uses profound HU.DWIM.WALKER system to do the heavy lifting of code walking and is, in fact, just a convenience wrapper around it.

Note: if your implementation is not supported by HU.DWIM.WALKER or CL-CURLEX, then branch "without-curlex" is for you - there initial unsophisticated behaviour of the system is fixed forever.

Transforming

Codewalk the form and transform undefined variables and functions in it on the fly to something else. Has a side effect of expanding all the macros in the form.

CL-USER> (ql:quickload 'cl-indeterminism)
CL-USER> (let ((cl-indeterminism:*variable-transformer* (lambda (x) `(quote ,x))))
           (cl-indeterminism:macroexpand-all-transforming-undefs '(a b c)))
(A 'B 'C)
CL-USER> (let ((cl-indeterminism:*function-transformer*
                (lambda (form)
                  (if (keywordp (car form))
                      (cl-indeterminism:fail-transform)
                      `(,(intern (string (car form)) "KEYWORD")
                        ,@(cdr form))))))
           (cl-indeterminism:macroexpand-all-transforming-undefs '(a b c)))
(:A B C)

The API consists of the following ingredients:

  • MACROEXPAND-ALL-TRANSFORMING-UNDEFS - macro, which actually does the transformation. Accepts optional keyword :ENV, which can be :NULL or :CURRENT (default) and acts the same as in FIND-UNDEFS.
  • *VARIABLE-TRANSFORMER* - NIL, or a reference to a function, which accepts variable name, and returns arbitrary lisp-form. This function is used to transform all undefined variables. If NIL, variables are left as-is, nothing happens. If non-NIL, the returned form is recursively walked, so all undefined variables and function in it are also expanded. Beware of infinite recursions, use FAIL-TRANSFORM to prevent it!
  • *FUNCTION-TRANSFORMER* - analogous to *VARIABLE-TRANSFORMER*, only handles transformation of undefined functions. When NIL, undefined functions are left as-is. When non-NIL, expected to accept entire form on undefined function call and return arbitrary lisp-form. The returned form is recursively walked, so all undefined variables and function in it are also expanded. Beware of infinite recursions, use FAIL-TRANSFORM to prevent it!
  • FAIL-TRANSFORM - a macro, which can be used inside functions, to which *VARIABLE-TRANSFORMER* and *FUNCTION-TRANSFORMER* are bound. When invoked, causes transformation not to be performed, that is undefined variable/function call is left as-is. Useful to prevent infinite recursions, as in example above, when transformation on unknown function is not performed if its name is already a keyword.

I personally plan to use all this machinery in my fork of ESRAP packrat parser (https://github.com/mabragor/esrap). There, the idea is to transform all undefined variable names inside DEFRULE macro into calls to functions, which try to continue parsing with help of a rule, corresponding to the name of undefined variable. See branch 'liquid' there for examples of usage of MACROEXPAND-ALL-TRANSFORMING-UNDEFS.

Transforming at compile-time

Works similar to transforming at runtime, but is far more interesting, isn't it?

CL-USER> (ql:quickload 'cl-indeterminism)
CL-USER> (defmacro autoquote (form &environment env)
           (let ((cl-indeterminism:*variable-transformer* (lambda (x) `(quote ,x))))
             (cl-indeterminism:macroexpand-cc-all-transforming-undefs form :env env)))
;; So, our AUTOQUOTE macro automatically quotes
;; all the undefined variables inside its form
CL-USER> (defun foo ()
           (autoquote (list b c)))
CL-USER> (foo)
(B C)
;; And it takes into account lexical environment
;; it is expanded in, so here B is not quoted
CL-USER> (let ((b 1))
           (defun foo ()
             (autoquote (list b c))))
CL-USER> (foo)
(1 C)

Note, that lexical environment has to be explicitly fetched with help of &ENVIRONMENT keyword of the enclosing macro and passed to MACROEXPAND-CC-ALL-TRANSFORMING-UNDEFS. This is due to my failure to fetch this compile environment in some implementations with some other tricks.

TODO:

  • (done) find undefined variables with respect to current lexenv, not null lexenv
    • (done) allow to specify null lexenv
  • (done) macro to manipulate undefined functions and variables conveniently

BUGS & GOTCHAS:

  • MACROLETs in the enclosing scope are not handled correctly

    CL-USER> (macrolet ((bar (b) nil)) (cl-indeterminism:find-undefs '(bar a)))
    ((:FUNCTIONS) (:VARIABLES A))
    

    although it should see that variable A is not really there (hence, it does not go into the body of BAR). This is due to the limitation of CL-CURLEX, where only names of functions, variabes and macros are captured, not their definitions.

  • HU.DWIM.WALKER, which is used in implementation of MACROEXPAND-ALL-TRANSFORMING-UNDEFS and MACROEXPAND-CC-ALL-TRANSFORMING-UNDEFS does not know about some declarations, specific to compilers, such as TRULY-DYNAMIC-EXTENT. This results in tons of style warnings on standard output. So, for now I simply kludged to muffle those. However, if you suspect that macroexpansion is done wrong, you maybe should enable those warnings again, look for clues, and then maybe send a patch :)