Thanks a lot for the detailed answer (comments below, main matter at the end). <div class="gmail_quote">On Sat, Jan 9, 2010 at 07:42, Eli Barzilay <<a href="mailto:eli@barzilay.org">eli@barzilay.org</a>> wrote: <blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div class="im">On Jan 8, Laurent wrote: > For example on trying to generalize the macro given by Carl, I often > run into problems like that: > > (define-syntax (define-shorthand2 stx) > > (define (->string x) ;; turn anything into a string > (format "~a" x)) > > (define-syntax-rule (combine-variable-names x ...) > (datum->syntax ;; from symbol to variable name > stx > (string->symbol ;; from string to symbol > (string-append ;; combine strings > (->string (syntax-e x)) ...)))) </div>Why is this a macro? Even with `defmacro', this makes more sense as just a function that appends a bunch of symbols. </blockquote><div> In my past attempts (on a different macro for a similar purpose), I had problems with `apply' (no #%app or #%top blablabla) so I thought the `...' might help. Now I guess I didn't use it with a `with-syntax', or the inner functions were outside of the macro body (I've just learned about the utility of inner functions in macros). </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> <div class="im"> </div>The problem there (among several others, like the unquoted `body', the fact that `new-id' and `long' are never used, etc.)</blockquote><div> Oh, yes, sorry about that, that was only a sketch to show the kind of problem. </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> is that `combine-variable-names' is just a macro (that is used inside a macro). If you don't use a macro, then you can make it with very simple code (I needed to guess what you're trying to do, because of those errors): <div class="im"> (define-syntax (define-shorthand stx) </div> (define (->string x) (format "~a" x)) (define (combine-variable-names xs) (datum->syntax stx (string->symbol (apply string-append (map ->string xs))))) (syntax-case stx () [(define-shorthand x ... expr) (with-syntax ([long (combine-variable-names (syntax->datum #'(x ...)))]) #'(define long expr))])) (define-shorthand x y z 123) xyz </blockquote><div> That's exactly what I was trying to do... (why didn't I re-try using `apply'! grrrr....) </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> If you insist on using a macro, then you can do this: [snip] </blockquote><div> A function is all what is needed. </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> But there is no way for this to work: (combine-variable-names #'x ...) because the `...' are only meaningful in `syntax' (or the #' shorthand). Another way to see the problem is that the `...' in the input pattern stands for a variable arity expression in the runtime code -- but in the syntax code, (combine-variable-names #'x ...) is just a function with two arguments. (Kind of.) </blockquote><div> Yes, that's why I tried to use it in a syntax and unsplice it like #`#,@(x ...) but that obviously means nothing. Now for the real problem: </div><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> <div class="im"> > P.S. : And that is very frustrating because it is so easy with > defmacro... I'm pretty sure many people are working hard to extend > nicely the syntax macro system, but I have much difficulty to use it > correctly and I find it cumbersome when you deal with non-trivial > macros. That's my major grief about Scheme. </div>Did you try to write it with `defmacro'? If you feel comfortable with it, it might be a good way to see the real problem. </blockquote><div> For the sake of it, here it is (yes, with `apply'): #lang scheme (require mzlib/defmacro) (define-macro (define-shorthand xs val) (define (->string x) (format "~a" x)) (define (combine-variable-names xs) (string->symbol (apply string-append (map ->string xs)))) (let ([new-id (combine-variable-names xs)]) `(define ,new-id ,val))) (define-shorthand (x y z) 123) xyz (Btw, isn't this macro hygienic?) My main problem is that `syntax-rules', `syntax-case', `with-syntax' and others that I have not used yet use the syntax in quite different ways, and sometimes you have to #' and sometimes you have to (syntax->datum ....) and sometimes not. I have a hard time to know when to use the right one at the right time (the docs are a bit obscure on these points). </div></div>Whereas in `defmacro', I know exactly when to use `quote' and `unquote'. Wouldn't it be possible to define "safe" quotes and unquotes for syntax macros, without having to go from syntax to datum and vice-versa at weird places? I mean, syntax objects are encapsulating other data that I really don't need 99% of the time. So why should I bother about something I don't want? Thanks again for your answers, Laurent