Can you use the procedural part of syntax-rules? You have the power of using scheme at expansion time, which means you could do list-ref all you want.
The only thing is that guile lacks syntax->list, so sometimes you have to manually turn it into a list. Say you are matching ((_ stuff ...) Body) stuff is a syntax object. You could turn it into a list of syntax objects by doing #'(stuff ...). Then you can treat it as a regular list, and use quasisyntax to put it back into your output syntax. Writing this on my phone. Sorry for the brevity (and lack of code). -- Linus Björnstam On Mon, 5 Apr 2021, at 13:30, Dr. Arne Babenhauserheide wrote: > Hi, > > In dryads-wake I need selection of the element in a list in a macro from > user-input. Currently I have multiple macros, and the correct one (which > strips the non-selected choices) is selected in a simple cond: > > (define-syntax-rule (Choose resp . choices) > "Ask questions, apply consequences" > (cond > ((equal? resp 1) ;; resp is user-input. It is a natural number. > (Respond1 choices)) > ((equal? resp 2) > (Respond2 choices)) > ((equal? resp 3) > (Respond3 choices)) > (else > #f))) > > For this however I have three syntax-case macros: > > (define-syntax Respond1 > (lambda (x) > (syntax-case x () > ((_ ((question consequences ...) choices ...)) > #`(begin > (respond consequences ...))) > ((_ (choices ...)) > #`(begin #f))))) > > (define-syntax Respond2 > (lambda (x) > (syntax-case x () > ((_ (choice choices ...)) > #`(begin > (Respond1 (choices ...)))) > ((_ (choices ...)) > #`(begin #f))))) > > (define-syntax Respond3 > (lambda (x) > (syntax-case x () > ((_ (a b choices ...)) > #`(Respond1 (choices ...))) > ((_ (choices ...)) > #`(begin #f))))) > > > I would like to get rid of those three definitions and replace them by > at most two (one that strips N initial list entries, and Respond1). > > I cannot move to procedures, because I have code that must be executed > only during final processing, and when I evaluate any of the > consequences (as it happens with procedure-arguments), then the timing > of the code execution does not match anymore. So I must absolutely do > this in macros. > > > I’ve tried to get that working, but all my tries failed. Is there a way > and can you show it to me? > > This is a minimal working example. The output should stay the same, > except for part 4, which needs this change to work (see at the bottom), > but I would like to: > > - replace Respond2 and Respond3 by something recursive, so resp can have > arbitrary high values (not infinite: max the length of the options) and > - replace the cond-clause by a call to the recursive macro. > > (define-syntax-rule (respond consequence consequence2 ...) > (begin > (write consequence) > (when (not (null? '(consequence2 ...))) > (write (car (cdr (car `(consequence2 ...)))))))) > > (define-syntax Respond1 > (lambda (x) > (syntax-case x () > ((_ ((question consequences ...) choices ...)) > #`(begin > (respond consequences ...))) > ((_ (choices ...)) > #`(begin #f))))) > > (define-syntax Respond2 > (lambda (x) > (syntax-case x () > ((_ (choice choices ...)) > #`(begin > (Respond1 (choices ...)))) > ((_ (choices ...)) > #`(begin #f))))) > > (define-syntax Respond3 > (lambda (x) > (syntax-case x () > ((_ (a b choices ...)) > #`(Respond1 (choices ...))) > ((_ (choices ...)) > #`(begin #f))))) > > > (define-syntax-rule (Choose resp . choices) > "Ask questions, apply consequences" > (cond > ((equal? resp 1) > (Respond1 choices)) > ((equal? resp 2) > (Respond2 choices)) > ((equal? resp 3) > (Respond3 choices)) > (else > #f))) > > > (display "Choose 1: should be bar:") > (Choose 1 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar)) > (newline) > (display "Choose 2: should be warhar:") > (Choose 2 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar)) > (newline) > (display "Choose 3: should be mar:") > (Choose 3 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar)) > (newline) > (display "Choose 4: should be tar:") > (Choose 4 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar)) > (newline) > (display "Choose 5: should be #f:") > (Choose 5 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar)) > (newline) > > > Best wishes, > Arne > -- > Unpolitisch sein > heißt politisch sein > ohne es zu merken > > Attachments: > * signature.asc