Thanks, that makes sense after looking very closely at the documentation. I was able to work around it by introducing a syntax-class, which had the added benefit of being more readable then nested ~or patterns.
-Eric On Fri, Jun 11, 2010 at 3:56 PM, Ryan Culpepper <ry...@ccs.neu.edu> wrote: > Eric Dobson wrote: >> >> I'm trying to use the syntax/parse library, and am running into a >> weird problem when using ~or. It works without ellipses, but with >> ellipses it complains about a duplicated attribute. I would assume >> that the attributes with the same name in an ~or pattern were the same >> attribute that could just match in different ways. A simplified >> version is below, the first 4 examples work, and the fifth does not. >> Is there a restriction that I am missing for why this does not work? > > There are three variants of ~or: single-term ~or, head ~or, and > ellipsis-head ~or. Ellipsis-head ~or has different rules for attributes, > which the docs seem not to explain. I'll update the docs. > >> (syntax-parse #'(1 2 3) >> [((~or x:number) ...) (syntax->datum #'(x ...))]) >> >> (syntax-parse #'(a b c) >> [((~or x:id) ...) (syntax->datum #'(x ...))]) > > Both of the above are ellipsis-head ~or, not that it matters. > >> (syntax-parse #'(1) >> [((~or x:id x:number)) (syntax->datum #'(x))]) >> >> (syntax-parse #'(a) >> [((~or x:id x:number)) (syntax->datum #'(x))]) > > These are both single-term ~or. > >> #; >> (syntax-parse #'(a 1 b) >> [((~or x:id x:number) ...) (syntax->datum #'(x ...))]) > > This is ellipsis-head ~or. The difference is that ellipsis-head ~or > accumulates different alternatives' attributes independently. The different > alternatives must have disjoint attributes. (For now, anyway; I'm > considering relaxing that restriction.) For example, > > (syntax-parse #'(a b #:c d e #:f g) > [((~or x:id (~seq k:keyword kx:id)) ...) > #'((x ...) ((k kx) ...))]) > => #<syntax ((a b e) ((#:c d) (#:f g)))> > > You can turn the ~or back into a single-term ~or by putting another pattern > form between it and the ellipses. For example, > > (syntax-parse #'(a 1 b) > [((~and (~or x:id x:number)) ...) ___]) > > Hope that helps! > > Ryan > > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
