Oh yea, An example of use: Define a layout of field types and lengths, then when creating a text line parser you give it the fields you want, and you get a (: Orderline-parser (String -> Orderline)) where Orderline is an implicitly created struct:
(define-fixed-layout Orderline-layout (order C 11) (order-sub C 5) (line C 7) (account C 10) (address-seq C 7) (create-date D 10) (order-type S 1) (location C 4) (dept C 2) (class C 3) (sku C 20) (vendor C 2) (entered-sku C 20) (source C 2) (list-price N 9) (sku-price N 10) (cost N 10) (uom C 2) (pack-qty I 7) (order-qty I 7) (ship-qty I 7) (misc-charge S 1) (sub-flag S 1) (contract-pp C 9) (contract-pp-seq C 5) (commission N 9) (contract-addendum C 1)) (define-static-parser-for-layout (Orderline-parser Orderline-layout) (order sku create-date order-type dept class)) On Wed, Jan 16, 2013 at 10:56 AM, Ray Racine <ray.rac...@gmail.com> wrote: > On the topic of "state" propagation across macros by extending the > expansion environment (or whatever the terminology would be). I have an > example where a macro is used to define a layout of a fixed offset text > file. Then a second macro expands into a mini field parser based on the > layout definition. Frankly my first macro beyond a basic syntax-rules > sort, so it's rough and assuredly less than optimal. It can be found here. > > > https://github.com/RayRacine/racketlib/blob/mapred/RpR/format/layout-types.rkt > https://github.com/RayRacine/racketlib/blob/mapred/RpR/format/layout.rkt > https://github.com/RayRacine/racketlib/blob/mapred/RpR/format/parser.rkt > > > On Wed, Jan 16, 2013 at 10:33 AM, Danny Yoo <d...@hashcollision.org>wrote: > >> On Wed, Jan 16, 2013 at 7:24 AM, Tim Brown <tim.br...@cityc.co.uk> wrote: >> > Folks, >> > >> > I would like to write a module that uses a macro to transform "define"s >> > and accumulate a value based on those transformations. >> > >> > In a fairly simple way, I want to do something like the below. However, >> it >> > seems the module-begin is being transformed before the my-defines. How >> do >> > I get counter to be incremented before I need it? >> >> The transformation happens top-down, so you'll see that the body of >> your program expands in the following steps (to a first >> approximation): >> >> >> (#%your-module-begin (your-define woo 2)) >> >> ==> >> >> (#%racket-module-begin (your-define woo 2) >> (define defines-counter 0) >> (provide defines-counter)) >> >> ==> >> >> (#%racket-module-begin (racket-define woo 2) >> (define defines-counter 0) >> (provide defines-counter)) >> >> >> One possible workaround is the the following: add one level of >> indirection in installing the value of the defines-counter. You can >> do something like: >> >> (define-syntax (get-counter stx) >> (datum->syntax stx counter)) >> >> and use (get-counter) in place of #,counter in your module-begin. >> This way, we delays the lookup of counter until the macro expander >> finally reaches that expression, and by that time, it should have hit >> all the defines already. >> >> >> If you need to control the order of expansion more explicitly, you may >> need to look into local-expand: >> >> >> http://docs.racket-lang.org/reference/stxtrans.html#(def._((quote._~23~25kernel)._local-expand)) >> >> ---- >> >> Modified code below: >> >> #lang racket/base >> >> (provide (except-out (all-from-out racket) #%module-begin define) >> (rename-out (module-begin #%module-begin) (my-define define))) >> >> (require (for-syntax racket/base >> syntax/parse)) >> (define-for-syntax counter 0) >> >> (define-syntax (my-define stx) >> (syntax-parse >> stx >> [(_ i v) (set! counter (add1 counter)) #`(define i v)] >> [(_ (i args ...) v ...+) (set! counter (add1 counter)) >> #`(define (i args ...) v ...)])) >> >> (define-syntax (get-counter stx ) >> (datum->syntax stx counter)) >> >> (define-syntax (module-begin stx) >> (syntax-parse >> stx >> [(_ expr ...) >> #`(#%module-begin expr ... >> (define defines-count (get-counter)) >> (provide defines-count))])) >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users >> > >
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