<div dir="ltr"><div><div><div>I think it isn't really predictable and you should always just use syntax->list. But in the first case, when you write a pattern like </div> (x y ...) </div>and you have an earlier syntax binding like you do, then the pattern matching macros see this as a 'cons'-like operation on syntax and so you get a pair of syntax objects. </div>Exploring the source locations of the intermediate "cons"es inside can also help shed light on what's happening any particular example, too. Robby <div> </div></div><div class="gmail_extra"> <div class="gmail_quote">On Mon, May 27, 2013 at 2:52 PM, Tim Nelson <<a href="mailto:tbnelson@gmail.com" target="_blank">tbnelson@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"> Thanks for the quick reply, and the helpful example! Can you say a bit more about the circumstances under which syntax-e will return a pair, even if its datum is a list? That's the biggest cause of confusion for me. For my purposes, I may just end up switching to syntax->list in my program, since that will always return a list. <div class="HOEnZb"><div class="h5"> On Mon, May 27, 2013 at 3:27 PM, Jay McCarthy <<a href="mailto:jay.mccarthy@gmail.com">jay.mccarthy@gmail.com</a>> wrote: > A syntax object contains a lot of information in addition to the datum > inside it. Sometimes that datum will be a pair or a list. And > sometimes the extra data needs to be associated with just the pair and > sometimes with all the elements of the list. > > If you look at this program: > > #lang racket/base > (require (for-syntax racket/base > racket/match)) > (begin-for-syntax > (define show-syntax > (match-lambda > [(? syntax? s) > (printf "syntax from line ~a\n" > (syntax-line s)) > (show-syntax (syntax-e s))] > [(? list? l) > (printf "list\n") > (for-each show-syntax l)] > [(? pair? p) > (printf "pair\n") > (show-syntax (car p)) > (show-syntax (cdr p))] > [(? symbol? s) > (printf "symbol ~a\n" s)]))) > > (define-syntax (ex stx) > (show-syntax stx) > #'(void)) > > (define-syntax (helper1 stx) > #`(ex #,stx)) > > (define-syntax (helper2 stx) > (syntax-case stx () > [(_ a ...) > (syntax > (ex a ...))])) > > (define-syntax (helper3 stx) > (syntax-case stx () > [(_ a ...) > (syntax > (ex a ... z))])) > > (module+ main > (ex x) > ex > (helper1 a b c) > (helper1 . (a b c)) > (helper2 a b c) > (helper3 a b c)) > > You can see how each of the different "helpers" appear to do things > that are very close to one another but turn out to have different > syntax representations. > > You normally don't need to worry about this, but sometimes you need to > pay attention to it when you want to get correct source code > origination information. (Basically, if you have a list in the pattern > input and do something like creating a new syntax object like #'(a > ...) in a macro and then put that in, then you'll get source code > location information from the macro and not from the source of the > list, which sometimes gets exposed.) > > Jay > > > On Mon, May 27, 2013 at 1:06 PM, Tim Nelson <<a href="mailto:tbnelson@gmail.com">tbnelson@gmail.com</a>> wrote: >> Dear All, >> >> The documentation for syntax-e says that it may return many different >> types, including a syntax-pair, but it doesn't really describe why. Here's >> a concrete piece of code: >> >> (define stx >> (with-syntax ([x #'5] >> [(y ...) #'(1 2)]) >> (syntax (x y ...)))) >> >> (syntax->datum stx) returns a list. >> >> (syntax->list stx) (of course) returns a list, too. >> >> But (syntax-e stx) returns a pair! If I do this: >> >> (define stx2 >> (with-syntax ([x #'5] >> [y #'(1 2)]) >> (syntax (x y)))) >> >> I get a list from syntax-e, but of course the nesting is wrong/different. >> >> So my question is: why a pair? >> >> Best, >> - Tim (Running Racket v. 5.3.3) >> ____________________ >> Racket Users list: >> <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> > > > > -- > Jay McCarthy <<a href="mailto:jay@cs.byu.edu">jay@cs.byu.edu</a>> > Assistant Professor / Brigham Young University > <a href="http://faculty.cs.byu.edu/~jay" target="_blank">http://faculty.cs.byu.edu/~jay</a> > > "The glory of God is Intelligence" - D&C 93 ____________________ Racket Users list: <a href="http://lists.racket-lang.org/users" target="_blank">http://lists.racket-lang.org/users</a> </div></div></blockquote></div> </div>