There's also this macro I wrote that I couldn't live without, I call it fmask. Problem: I always hated that (loop for element in list collect (my-fun element))

There's also this macro I wrote that I couldn't live without, I call it fmask. Problem: I always hated that (loop for element in list collect (my-fun element)) translates trivially to (mapcar #'my-fun list) but (loop for element in list collect (my-fun element my-constant)) doesn't. You'd either have to stick with the loop version or use something ugly and inefficient like (mapcar #'my-fun list (make-list (length list) :initial-element my-constant)) or write out the lambda: (mapcar (lambda (element) (my-fun element my-constant)) list) (my-constant could actually be an arbitrary form). None of these solutions appealed to me. So I wrote fmask. With it, the example is simply rewritten as (mapcar (fmask #'my-fun ? (? my-constant)) list) Another example: (mapcar (fmask #'list ? (? (1+ (length ?)))) '(3 8 2) '(("some" "items") "A string" (nil "test"))) =>((3 3) (8 9) (2 3)) Here's the implementation: (defmacro fmask (function unknown (&rest args)) (check-type unknown symbol) (macrolet ((autotraverse-let (bindings &body body) (let ((gensyms (iter (repeat (length bindings)) (collect (gensym "TRAVERSED"))))) `(let (,@(iter (for (nil value) in bindings) (for gensym in gensyms) (collect `(,gensym ,value)))) (symbol-macrolet (,@(iter (for (var nil) in bindings) (for gensym in gensyms) (collect `(,var (prog1 (car ,gensym) (setf ,gensym (cdr ,gensym))))))) ,@body))))) (labels ((unknownp (arg) (etypecase arg (atom (eq arg unknown)) (cons (some #'unknownp arg)))) (literalp (arg) (or (typep arg '(or array keyword pathname number)) (and (consp arg) (or (and (eq (first arg) 'function) (cdr arg) (not (cddr arg))) (eq (first arg) 'quote)))))) (let ((unknowns (iter (repeat (count-if #'unknownp args)) (collect (gensym "UNKNOWN")))) (knowns (iter (for arg in args) (if (not (or (unknownp arg) (literalp arg))) (collect (list (gensym "KNOWN") arg))))) (known-function (and (consp function) (eq (first function) 'function) (symbolp (second function)) (second function)))) (autotraverse-let ((an-unknown unknowns) (a-known (mapcar #'first knowns))) (let* ((unknown-function (gensym "UNKNOWN-FUNCTION")) (call-function (if known-function (list known-function) `(funcall ,unknown-function))) (main (let ((inner `(lambda (,@unknowns) (,@call-function ,@(iter (for arg in args) (collect (if (unknownp arg) (subst an-unknown unknown arg) (if (literalp arg) arg a-known)))))))) (if knowns `(let (,@knowns) ,inner) inner)))) (if known-function main `(let ((,unknown-function ,function)) ,main))))))))