Wow, section 4 is exactly, almost word for word, what I was trying to do. This is a great resource.
On Fri, May 3, 2013 at 7:35 AM, Philipp Dikmann <phil...@dikmann.de> wrote: > You might have already read it, but I also found the guide "Fear of > Macros" by Greg Hendershott incredibly helpful in understanding them, > especially considering things like with-syntax and format-id: > http://www.greghendershott.com/fear-of-macros/index.html > > > On 03.05.13 04:57, Sean McBeth wrote: > > 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 >>> >> > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users > > >
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