Ok, now I am confused, which is not a surprise, really. As far as I understand this - the problem is that when specifying yield in the original gets 'renamed' if the macro is used in other macros due to hygiene rules. Ryan's suggestion is to use syntax-parameter, which is a annotation living in the syntax environment only, so it is not altered by the hygiene rules. What is the utility of a syntax-parameter? A syntax variable? Matthias's solution is to augment the body0 syntax-object with the yield binding not the name. Since name might be a used in other contexts, so the resulting yield is identified to be used in the precisely right context. Hygiene rules won't do the wrong renames/tagging. So you could define a kind of a rule of thumb - use the inner most possible scope to introduce new identifiers. Cheers, Vlado <div>On 8/4/07, Matthias Felleisen <<a href="mailto:matthias@ccs.neu.edu">matthias@ccs.neu.edu</a>> wrote:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> Vlado, you can, however, choose a different piece of context syntax for the introduction of the yield binding, namely, one from the scope you really want:   (define-syntax (define/y stx)    (syntax-case stx ()      [(_ (name arg ...) body0 body ...)       (with-syntax ((yield-name (datum->syntax-object #;stx (syntax body0) 'yield)))         (syntax          (define (name arg ...)            (define (yield-name x)              (control resume-here               (set! name                     (lambda ()                       (prompt (resume-here 'dummy))))               x))            (prompt body0 body ...))))])) If you now target this mini-language with a macro, the non-hygienic binding is introduced properly, e.g.,    (define-syntax foo      (syntax-rules ()       ((_ (name arg ...) body ...) (define/y (name arg ...) body ...))))    (foo (bar) (printf "hello world\n") (yield 1) (yield 2) 'finished)    (list (bar) (bar) (bar) (bar)) -- Matthias On Aug 4, 2007, at 10:58 AM, Ryan Culpepper wrote: > On Fri, 2007-08-03 at 13:35 +0100, Vladimir Zlatanov wrote: >> >>         2. How can I use the macro system to eliminate the >>         'yield-name' part >>         of the macro? >> >> One solution that works, but I do have a few problems with it. >> I had to break the hygiene, which in this case might not be that bad, >> but it is not nice. More worringly for me, I'm not sure why does it >> really work the way it works. It is a problem with my >> understanding of >> macros, but that might pass with time =) >> >> What would be a hygienic solution? > > One problem with using 'datum->syntax-object' to introduce identifiers > (breaking hygiene) is that it often breaks down when you write another > macro that expands into the first one. > > For example, here's a trivial macro: > > (define-syntax define-gen >   (syntax-rules () >     [(define-gen (name . args) . body) >      (make-gen (name . args) . body)])) > > Then use it: > > (define-gen (make-step) >   (yield 1) >   (yield 2) >   (yield 3) >   'finished) > (define step (make-step)) > (step) ;;  produces error: undefined identifier 'yield' > > Why? The 'make-gen' identifier produced by 'define-gen' had a mark on > it, so the introduced 'yield' identifier had a mark on it, so it > didn't > bind the original, unmarked occurrences of 'yield'. > > One nice way to solve this problem is to use syntax parameters. There > was a thread on this list about them about a year ago, and I wrote > up a > summary here: > > <a href="http://macrologist.blogspot.com/2006/04/macros-parameters-binding-">http://macrologist.blogspot.com/2006/04/macros-parameters-binding- </a> > and.html > > There's more information in the Help Desk: search for > 'syntax-parameterize'. > > Ryan > > >> >> =========================== >> (require (lib "control.ss")) >> >> (define-syntax (make-gen stx) >>   (syntax-case stx () >>     [(mkg (name arg ...) body ...) >>      (syntax-case (datum->syntax-object (syntax mkg) 'yield) () >>        [yield (syntax >>                (define (name arg ...) >>                  (define (control-state) body ...) >>                  (define (yield value) (control resume-here (set! >> control-state resume-here) value)) >>                  (lambda () (prompt (control-state)))))])])) >> >> (make-gen (make-step) >>           (yield 1) >>           (yield 2) >>           (yield 3) >>           'finished) >> >> (define step (make-step)) >> >> (step)(step)(step)(step)(step) >> >> =========================== >> >> _________________________________________________ >>   For list-related administrative tasks: >>   <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme">http://list.cs.brown.edu/mailman/listinfo/plt-scheme </a> > > _________________________________________________ >   For list-related administrative tasks: >   <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme">http://list.cs.brown.edu/mailman/listinfo/plt-scheme </a> </blockquote></div>