try `syntax/parse/experimental/template`. It gives you access to `??', which basically says "use a if present, else b", and some other cool templates besides. From the docs:
> (syntax-parse #'(m 1 2 3) [(_ (~optional (~seq #:op op:expr)) arg:expr ...) (template ((?? op +) arg ...))]) #<syntax:197:0 (+ 1 2 3)> > (syntax-parse #'(m #:op max 1 2 3) [(_ (~optional (~seq #:op op:expr)) arg:expr ...) (template ((?? op +) arg ...))]) #<syntax:198:0 (max 1 2 3)> On Wed, May 27, 2015 at 12:03 AM Alexis King <lexi.lam...@gmail.com> wrote: > When using syntax/parse, is there a good way to do something like this? > > (define-splicing-syntax-class options > (pattern (~seq (~or (~optional (~seq (~and #:a a?))) > (~optional (~seq (~and #:b b?))) > (~optional (~seq (~and #:c c?))) > (~optional (~seq (~and #:d d?)))) > ...) > #:attr a/b #'(a? b?) > #:attr c/d #'(c? d?))) > > When using the above syntax class to parse #'(#:a #:d #:b), then the a/b > attribute should be #'(#:a #:b) and the c/d attribute should be #'(#:d). > However, this doesn't work, of course, because if one of the options isn't > defined, the attribute will be #f, and the attribute binding will fail. > > I can get around that by doing something like this: > > (define-splicing-syntax-class options > (pattern (~seq (~or (~optional (~seq (~and #:a a?))) > (~optional (~seq (~and #:b b?))) > (~optional (~seq (~and #:c c?))) > (~optional (~seq (~and #:d d?)))) > ...) > #:attr a/b #`(#,@(if (attribute a?) #'(a?) #'()) > #,@(if (attribute b?) #'(b?) #'())) > #:attr c/d #`(#,@(if (attribute c?) #'(c?) #'()) > #,@(if (attribute d?) #'(d?) #'())))) > > But that's rather long-winded and verbose. Even better would be a way to > group the clauses within the pattern, something like this: > > (define-splicing-syntax-class options > (pattern (~seq (~or (~and (~seq (~optional (~seq (~and #:a a?))) > (~optional (~seq (~and #:b b?)))) > a/b) > (~and (~seq (~optional (~seq (~and #:c c?))) > (~optional (~seq (~and #:d d?)))) > c/d)) > ...))) > > But that obviously doesn't work, and I'm not sure what it would do even if > it compiled. > > Anyway, is there a more concise way to do this? I know it's relatively > easy using `template` from syntax/parse/experimental/template, but this is > going into the Typed Racket code, so we're intentionally avoiding a > dependency on that. > > Alexis > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
