Now I've been trying a few times to generalize such a macro, because I occasionnaly need something like that, but with not much success. At best, I'd like something that I could use like this : (define-syntax-rule (define-identifier name-piece ... val) (with-new-identifier (new-id name-piece ...) (define new-id val))) The with-new-identifier macro would construct a name from name-pieces (strings, symbols, numbers, whatever simple values), so that the previous macro could assign it a value. 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)) ...)))) ;; here's the meat of the macro: (syntax-case stx () [(define-shorthand2 (new-id x ...) body ...) (with-syntax ([long (combine-variable-names #'x ...)]) ; PROBLEM body ...)])) The problem is with the ellipses that won't be included properly. I also tried (and failed) using #'... or #`#,@(x ...) just to see, without being too sure about what that meant. Could I have a hint on how I could do that? Thanks, Laurent 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 class="gmail_quote">2010/1/8 Carl Eastlund <<a href="mailto:cce@ccs.neu.edu">cce@ccs.neu.edu</a>> <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 Fri, Jan 8, 2010 at 8:55 AM, <<a href="mailto:mike@goblin.punk.net">mike@goblin.punk.net</a>> wrote: > </div><div><div></div><div class="h5">> Thank you for the explanation. At the time, I was just > exploring what I could do with Scheme. I am still trying to > think Scheme-ishly. I haven't learned macros yet, but maybe > that should move up on my agenda. > > Scheme seems to have more levels of indirection than what I'm > used to. Returning to my (contrived) problem, how might I force > the evaluation of a symbol? For example, the apply expression > below will evaluate date-year and then apply the procedure to > a-date, but the subsequent map expression will get the symbol > 'date-year and try to apply that to a-date. > > Or is this approach a dead end in Scheme? > > (define selectors > (map (ë (a-symbol) (string->symbol (string-append "date-" (symbol->string a-symbol)))) > '(year > month > day > week-day > hour > minute > second))) > > (apply date-year (list a-date)) > > (map (ë (x) (apply x (list a-date))) > selectors) > > Thanks again, > MJG > > P.S. I've changed my subscription details to try to avoid > triggering the spam filter. </div></div>Definitely time to move macros up on your agenda. Evaluating a run-time symbol to a run-time function is tricky... it's really no easier than evaluating a run-time number to a run-time function. Run-time symbols are just symbols, they don't have another meaning. Compile-time symbols, on the other hand, are used as variable names in compiled code. They can easily correspond to run-time functions: (define (f x) (+ x 1)) f ;; <-- compile-time symbol, run-time function Macros let you compute with compile-time symbols and construct variables, so they let you play games like the renaming you want to do. Here's some freebie code: -------------------------------------------------- #lang scheme ;; define a macro for binding one shorthand: (define-syntax (define-shorthand stx) ;; helper function for tedious conversion of compile-time datatypes... (define (combine-variable-names prefix suffix) (datum->syntax ;; from symbol to variable name stx (string->symbol ;; from string to symbol (string-append ;; combine strings (symbol->string (syntax-e prefix)) ;; from variable name to symbol to string "-" (symbol->string (syntax-e suffix)))))) ;; here's the meat of the macro: (syntax-case stx () [(define-shorthand prefix suffix) ;; pull apart the macro's inputs (with-syntax ([long (combine-variable-names #'prefix #'suffix)]) ;; combine the names #'(define suffix long))])) ;; construct the resulting definition ;; define another macro for several shorthands at once: (define-syntax-rule (define-shorthands prefix [suffix ...]) (begin (define-shorthand prefix suffix) ...)) ;; now try it out: (define-shorthands date [year month day week-day hour minute second]) (define now (seconds->date (current-seconds))) (year now) (month now) (day now) (week-day now) (hour now) (minute now) (second now) -------------------------------------------------- This is not a particularly easy macro to write. It turns out combining multiple variable names (such as date + year = date-year) is a bit tedious, and not supported in the simplest form of macros. The simple ones just shuffle around existing names, without creating new ones. Still, it's an alternative to using eval, and maybe a starting point for finding out more about macros. Good luck and happy Scheming! --Carl <div><div></div><div class="h5">_________________________________________________ For list-related administrative tasks: <a href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme" target="_blank">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> </div></div></blockquote></div>