(You have reimplemented my macros from 2htdp/universe :-)
On Aug 17, 2012, at 2:29 PM, Walter Tetzner wrote: > Here's one that gives you keyword arguments, and builds a Racket > struct. If you don't give a value for a field, it defaults to #f. It > also prints with the keyword arguments. > > There's more code than Matthias' example, though. :) > > Also, I might write a patch to have `struct' produce a constructor > that accepts keyword arguments. It would either generate a new > constructor that took keyword arguments, or it would have the current > constructor also be able to take keyword arguments. I also think having > fields default to #f is probably a bad idea. So, 1) is anyone > interested in this, and 2) which approach would be better? > > #lang racket > (require (for-syntax racket/syntax > racket)) > > ;; > ----------------------------------------------------------------------------- > ;; library code: > (define-for-syntax (id->keyword id) > (datum->syntax #'id > (string->keyword > (symbol->string > (syntax->datum id))))) > > (define-for-syntax (id->keyword-arg id default) > (list (id->keyword id) > (list (generate-temporary (syntax->datum id)) default))) > > (define-for-syntax (ids->keyword-args ids default) > (append-map (λ (id) (id->keyword-arg id default)) > (syntax->list ids))) > > (define-for-syntax (struct-print id make-id obj fields) > (let ([fields (syntax->list fields)]) > (with-syntax ([format-str (format "(~a ~a)" > (syntax->datum make-id) > (string-join (map (λ (field) > (format "#:~a ~~s" > (syntax->datum field))) > fields) " "))]) > #`(format format-str #,@(map (λ (field) > (list (format-id id "~a-~a" id field) > obj)) > fields))))) > > (define-syntax (defstruct stx) > (syntax-case stx () > [(defstruct id field ...) > (let* ([key-args (ids->keyword-args #'(field ...) #'#f)]) > (with-syntax* ([make-id (format-id #'id "make-~a" #'id)] > [(arg ...) (map (λ (item) > (first item)) > (filter list? key-args))]) > #`(begin > (struct id (field ...) > #:transparent > #:methods gen:custom-write > [(define write-proc > (λ (obj port mode) > (write-string #,(struct-print #'id #'make-id > #'obj #'(field ...)) > port)))]) > (define (make-id #,@key-args) > (id arg ...)))))])) > > ;; > ----------------------------------------------------------------------------- > ;; client code > (defstruct person name age waist-size favorite-color) > > (define p (make-person #:age 35 #:favorite-color "blue" #:name "Bob")) > > (person-age p) > (person-waist-size p) > > -Walter > > On Fri, Aug 17, 2012 at 8:58 AM, Matthias Felleisen > <[email protected]> wrote: >> >> p.s. Here is a naive macro that gets you close, assuming you're willing to >> expand to classes: >> >> #lang racket >> >> ;; >> ----------------------------------------------------------------------------- >> ;; library code: >> (define-syntax-rule >> (defstruct s f ...) >> (begin (define x (class object% (init-field (f '()) ...) (super-new))) >> (define-syntax-rule >> (s a (... ...)) >> (new x a (... ...))))) >> >> ;; >> ----------------------------------------------------------------------------- >> ;; client code >> (defstruct person name age waist-size favorite-color) >> >> (define p (person (age 35) (favorite-color "blue") (name "Bob"))) >> >> (get-field age p) >> (get-field waist-size p) >> >> >> >>
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