I appreciate the help, but I'll point out that I explicitly said I didn't want to use `template` in my original message. ;)
> On May 27, 2015, at 05:18, Spencer Florence <[email protected]> wrote: > > 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 <[email protected]> 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 [email protected]. For more options, visit https://groups.google.com/d/optout.
