On Sun, Nov 14, 2010 at 9:57 PM, Jon Rafkind <rafk...@cs.utah.edu> wrote: > On 11/14/2010 07:52 PM, Sam Tobin-Hochstadt wrote: >> On Sat, Nov 13, 2010 at 5:09 PM, D Marshall <dmarshall...@gmail.com> wrote: >>> I understand that DATUM->SYNTAX gets it's context >>> information from it's first argument. I have no problems >>> when using simple (non-ellipses) pattern variables, however, >>> I haven't been unable to get a strictly "ellipses pattern" >>> to work in this context (no pun intended). >> Here are two options: >> >> This is my preference - use the context of the macro application >> (define-syntax test-this >> (lambda (x) >> (syntax-case x () >> [(_ a ...) >> (with-syntax >> ([form (datum->syntax x >> `(+ ,@(syntax->datum (syntax (a ...)))))]) >> (syntax form))]))) >> >> This is also common, but can break in some cases: >> (define-syntax test-this >> (lambda (x) >> (syntax-case x () >> [(kw a ...) >> (with-syntax >> ([form (datum->syntax (syntax kw) >> `(+ ,@(syntax->datum (syntax (a ...)))))]) >> (syntax form))]))) >> >> The reason what you tried to do didn't work is that you need a context >> that is from the context of the macro use. The rest of the list isn't >> something the programmer originally wrote, but something that was >> generated by your macro, so it doesn't have any useful context. > This works too: > > (define-syntax test-this > (lambda (x) > (syntax-case x () > [(_ a ...) > (with-syntax > ([form (datum->syntax (car (syntax->list #'(a ...))) > `(+ ,@(syntax->datum #'(a ...))))]) > (syntax form))])))
Well, sometimes: > (test-this) car: expects argument of type <pair>; given '() -- sam th sa...@ccs.neu.edu _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
