Re: Syntax-Case macro that selects the N-th element from a list

["Linus Björnstam" <linus.internet@fastmail.se>]

That "syntax-rules" is of course syntax-case.

Try writing it first with unhygienic macros and get that working before porting to syntax-case if you don't know the ins-and-outs of syntax-case.

-- 
  Linus Björnstam

On Mon, 5 Apr 2021, at 14:21, Linus Björnstam wrote:
> 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
> 
>