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.
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