Sorry, I was combining all the examples into one for brevity. x has 2 possible bindings (one in the module and one in the macro), y is bound in the module and closes over the macro, and z is unbound and therefore should result in an error since nothing can be done with it.
;; example 1 (as per our earlier discussion) (define-dsl-syntax (pred? x) (or (= 2 x) (> x 3))) (recursive-expand (pred? 1)) '(or (= 2 1) (> 1 3)) ;; example 2 (capturing a reference bound by the enclosing environment) (define y 3) (define-dsl-syntax (pred? x) (or (= 2 x) (> x y))) (recursive-expand (pred? 1)) '(or (= 2 1) (> 1 3)) ;; example 3 (ensuring some degree of hygiene, note the two bindings for x) (define x 3) (define y 3) (define-dsl-syntax (pred? x) (or (= 2 x) (> x y))) (recursive-expand (pred? 1)) '(or (= 2 1) (> 1 3)) ;; example 4, (I'll throw in this case, or replace the quoted output with ??? for each unbound id (define-dsl-syntax (pred? x) (or (= 2 x) (> x z))) (recursive-expand (pred? 1)) >> unbound identifier: z at pred? ;; or maybe '(or (= 2 1) (> 1 ???)) Just to rephrase, my "expansion" is essentially just a "procedure->string" operation, or a custom-write, that I will use to target sql or something else. Any use of pred? at runtime within racket will run fine as is. Thanks, Scott. On Sun, Jan 26, 2014 at 2:47 PM, Carl Eastlund <carl.eastl...@gmail.com>wrote: > Scott -- I don't understand exactly what you're asking, at least not based > on the example you wrote. You defined x and y, but the error is about z. > Is this just a typo? Or are you expecting a value for z to come from > somewhere? > > Carl Eastlund > > > On Sun, Jan 26, 2014 at 1:01 PM, Scott Klarenbach <sc...@pointyhat.ca>wrote: > >> Thanks a lot Carl...this is very enlightening. If I could impose one >> last question before I go off and digest everything: >> >> What is the "correct" approach to capturing the runtime values of any >> references that may be bound by the enclosing environment, for splicing >> into the final recursively-expanded expression? >> >> ;; example 1 >> >> (define y 3) >> (define x 3) >> (define-dsl-syntax (pred? x) (or (= 2 x) (> x y))) >> (define-dsl-syntax (bad-pred? x) (or (= 2 x) (> x z))) >> >> (recursive-expand (pred? 1)) >> '(or (= 2 1) (> 1 3)) >> >> (recursive-expand (bad-pred? 1)) >> >> unbound identifier: z >> >> My naive approach was to collect all the identifiers in the expression >> body that had bindings, compare them to the argument list of the macro with >> bound-identifier=? to see which ones were explicitly introduced by the >> user, and then eval the remaining ones at runtime in a second step in order >> to splice them in. >> >> I haven't tried this, and am sure people are cringing just by reading it, >> lol. I know there are tons of features like marking and syntax properties >> and origins, etc which I don't yet understand, and which may provide a more >> durable solution. >> >> If needbe, I could explicitly provide to the macro the bindings I wish to >> capture, like postgresql does with query params...ie, >> >> (define-dsl-syntax (pred? x) (or (= 2 x) (> x $1)) #:capture (y)) ;; or >> something >> >> but for obvious reasons it is much better if these expressions just >> expanded and automatically captured any referenced values in the same way >> as would happen at runtime. >> >> Thanks a lot. >> >> Scott. >> >> >> >> >> On Sat, Jan 25, 2014 at 6:00 PM, Carl Eastlund >> <carl.eastl...@gmail.com>wrote: >> >>> Scott, >>> >>> I see what you're doing now. You're not actually trying to use macro >>> expansion at all; you're just using local-expand to substitute the >>> definition of pred? where it occurs, so that you can make its macro >>> definition also serve as its DSL definition. That's sensible, but >>> local-expand is still doing more than you want it to. That's why I put in >>> all the expansion caveats -- not because you necessarily meant to do full >>> expansion, but because local-expand is pretty explicitly built for full >>> expansion, and always tries to push as far as it can. Any time the caveats >>> about expansion don't apply, local-expand is probably a bigger gun than you >>> need. >>> >>> Where local-expand is going to bite you is when the definition of pred? >>> uses a macro at its top level. For instance: >>> >>> (define-syntax-rule (pred? x) (or (< x 3) (> x 7))) >>> >>> Here, local-expand is going to expand the use of (or ...), and any macro >>> that (or ...) produces at its top level, until you reach a core form as the >>> main expression, or something you've put in an explicit stop list. That's >>> not what you want, as I understand it -- you only want to expand pred?. >>> >>> So what to do when you want to apply one macro, but not perform general >>> expansion? Extract its transformer using syntax-local-value, and apply it >>> to the expression. You probably also want to apply a syntax mark before >>> and after transformation, just to simulate the base level of hygiene the >>> macro may be relying on. It might not be necessary for simple definitions, >>> but it can't hurt. >>> >>> I wrote up some code that does this, along with a test showing that it >>> won't expand "or" too far. It's also reasonably hygienic -- it won't be >>> confused if someone defines a different macro named "pred?", for example. >>> I don't know if that's a concern, but again, it can't hurt. Anyway, you >>> can find what I wrote here: >>> https://gist.github.com/carl-eastlund/8626893 >>> >>> Carl Eastlund >>> >>> On Fri, Jan 24, 2014 at 1:30 PM, Scott Klarenbach <sc...@pointyhat.ca>wrote: >>> >>>> Just an update, I was able to make this work. >>>> >>>> #lang racket >>>> (require (for-syntax racket/syntax syntax/stx)) >>>> >>>> (define-syntax-rule (pred? x) (> 3 x)) >>>> >>>> (define-for-syntax (recursive-expand stx) >>>> (let loop ([l (syntax->list stx)]) >>>> (cond [(stx-null? l) l] >>>> [(stx-pair? (stx-car l)) >>>> (cons (loop (stx-car l)) (loop (stx-cdr l)))] >>>> [(equal? 'pred? (syntax->datum (stx-car l))) >>>> (local-expand (cons (stx-car l) (loop (stx-cdr l))) 'expression >>>> #f)] ;; this works >>>> [else >>>> (cons (stx-car l) (loop (stx-cdr l)))]))) >>>> >>>> (define-syntax (test stx) >>>> (syntax-case stx () >>>> [(_ x) >>>> (with-syntax ([expanded (recursive-expand #'x)]) >>>> #''expanded)])) >>>> >>>> (module+ test >>>> (require rackunit) >>>> (check-equal? (test (or (< 10 x) (pred? y))) >>>> '(or (< 10 x) (> 3 y)))) >>>> >>>> The code I couldn't figure out last night was: >>>> (local-expand (cons (stx-car l) (loop (stx-cdr l))) 'expression #f)] >>>> >>>> Thanks. >>>> >>>> -- >>>> Talk to you soon, >>>> >>>> Scott Klarenbach >>>> >>>> PointyHat Software Corp. >>>> www.pointyhat.ca >>>> p 604-568-4280 >>>> e sc...@pointyhat.ca >>>> 200-1575 W. Georgia >>>> Vancouver, BC V6G2V3 >>>> >>>> _______________________________________ >>>> To iterate is human; to recur, divine >>>> >>> >>> >> >> >> -- >> Talk to you soon, >> >> Scott Klarenbach >> >> PointyHat Software Corp. >> www.pointyhat.ca >> p 604-568-4280 >> e sc...@pointyhat.ca >> 200-1575 W. Georgia >> Vancouver, BC V6G2V3 >> >> _______________________________________ >> To iterate is human; to recur, divine >> > > -- Talk to you soon, Scott Klarenbach PointyHat Software Corp. www.pointyhat.ca p 604-568-4280 e sc...@pointyhat.ca 200-1575 W. Georgia Vancouver, BC V6G2V3 _______________________________________ To iterate is human; to recur, divine
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