I think I get it just from reading it (in bed, on the phone, annoying the wife). I had tried to do almost this very thing with datum->syntax at one point, but I had put the quotesyntax on datum->syntax, not on id directly. I don't understand why that would make a difference, it seems like it is similar to doing (list 'a 'b) instead of '(a b)... oh, nope, now I get it. It specifically *is* similar, except my second example should have been '(list a b). On May 2, 2013 10:38 PM, "Sean McBeth" <sean.mcb...@gmail.com> wrote:
> Aaah, man. Thanks. Shutdown the pc for the night so u will try tomorrow. > On May 2, 2013 10:35 PM, "Jay McCarthy" <jay.mccar...@gmail.com> wrote: > >> You were close to what you want. Here's a version with a nice utility >> and then the underlying machinery that makes it: >> >> #lang racket >> (require (for-syntax racket/syntax)) >> >> (define-syntax (double-define stx) >> (syntax-case stx (double-define) >> [(_ id val1 val2) >> (with-syntax ([id-1 (format-id #'id "~a-1" #'id)] >> [id-2 (datum->syntax >> #'id >> (string->symbol >> (format "~a-2" >> (syntax->datum >> #'id))))]) >> #'(begin (define id-1 val1) >> (define id-2 val2)))])) >> >> (double-define id 3 7) >> (displayln id-1) >> (displayln id-2) >> >> On Thu, May 2, 2013 at 8:29 PM, Sean McBeth <sean.mcb...@gmail.com> >> wrote: >> > Hi there! >> > >> > I'm pretty new to Racket, though not the basic concepts of functional >> > programming [1] Maybe I don't need macros here at all, but it seemed >> like >> > the right sort of lever when I first started, but now I'm pretty >> stuck[2] >> > and I don't understand enough about the macro system yet to be able to >> > figure this out. >> > >> > Basically, I'm trying to make a database migration tool + relational >> mapper. >> > I'd like to be able to define my tables in an abbreviated Racket syntax >> and >> > use the definition to generate everything from the create-table SQL >> scripts, >> > a few, basic CRUD-scripts-for-all-columns to structs that will mirror a >> full >> > table row when processing the query results. >> > >> > Right now, the table definition looks like this: >> > >> > (define-table tickets get-all-tickets >> > ([ticket_id serial primary-key] >> > [priority int nullable] ;; I believe in making not-null the default >> case >> > [description (varchar max)] >> > [finished_on datetime (default "9999-12-31 23:59:59.999")]) >> > >> > And this is pretty easy to parse into some "table" structs that describe >> > everything fairly sufficiently[3]: >> > https://gist.github.com/capnmidnight/5506674 >> > >> > Now, my sticking point is that I don't want to have explicitly define >> that >> > "get-all-tickets" identifier. I notice that, in my creating the "column" >> > struct, I've received a number of procedures for the constructor and >> field >> > accessors, all given a prefix of "column" for their identifier. So at >> first >> > glance, it seems like there are forms like struct that are capable of >> > dynamically defining identifiers. >> > >> > So, I stepped into the definition for struct and tried to make sense of >> it, >> > but the best I could figure out was that struct used syntax-case >> instead of >> > syntax-rules. It was a bit of a hair-ball for me, I couldn't suss out >> the >> > cross references, and at least at this late of an hour I'm having >> trouble >> > understanding the documentation on syntax-case. >> > >> > Specifically, I tried to do something like: >> > >> > (define-syntax (double-define stx) >> > (syntax-case stx (double-define) >> > [(_ id val1 val2) >> > #`(begin (define id-1 val1) >> > (define id-2 val2))])) >> > >> > (double-define id 3 7) >> > (displayln id-1) ;; error "id-1 unbound identifier" >> > (displayln id-2) >> > >> > I then tried something like: >> > >> > (define-syntax (double-define stx) >> > (syntax-case stx (double-define) >> > [(_ id val1 val2) >> > (with-syntax ([id-1 #'(string->symbol (format "~a-1" id))] ;; error >> > "define: not an identifier, identifier with default, or keyword for >> > procedure argument" >> > [id-2 #'(string->symbol (format "~a-2" id))]) >> > #'(begin (define id-1 val1) >> > (define id-2 val2)))])) >> > >> > (double-define id 3 7) >> > (displayln id-1) >> > (displayln id-2) >> > >> > Clearly, not correct. >> > >> > I could make the table struct into a table class and then just define a >> > get-all method that does what I want, but that kind of feels like >> giving up >> > and I'm more interested in using this to learn more about using macros, >> as >> > it has already done for me. >> > >> > >> > >> > [1] Functional C# is something of a job safety program of mine :P >> > >> > [2] i.e. been banging my head against the desk for the last 6 hours. I >> have >> > gotten pretty comfortable with syntax-rules though, so it wasn't a >> complete >> > waste. >> > >> > [3] This isn't the final form, but I'm just pushing some code around to >> try >> > to get the basic concepts working. For example, the get-all-tickets >> > procedure wouldn't just return the query, it'd eventually execute it and >> > return the results. >> > >> > ____________________ >> > Racket Users list: >> > http://lists.racket-lang.org/users >> > >> >> >> >> -- >> Jay McCarthy <j...@cs.byu.edu> >> Assistant Professor / Brigham Young University >> http://faculty.cs.byu.edu/~jay >> >> "The glory of God is Intelligence" - D&C 93 >> >
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