[racket] structure question

On Wed, 8 Dec 2010 14:39:52 -0500
Matthias Felleisen <matthias-1vnkWVZi4QaVc3sceRu5cw at public.gmane.org>
wrote:

> 
> 
> 
> #lang racket
> 
> ;;
> ----------------------------------------------------------------------------- ;;
> library module 
> 
> ;; syntax def
> ;; definition = ... | (struct/kw name (field ...) options ...)
> ;; meaning:           (struct name (field ...) options ...) 
> ;;                     plus keyword-based constructor: name/kw
> ;; WARNING: the introduction of name/kw is non-hygienic 
> ;; NEW: allow optional default expressions on field-s
> ;; WARNING: this new option will interfere with the #:auto field
> option
> 
> (define-syntax (struct/kw stx)
>   (syntax-case stx ()
>     [(_ name (field ...) . stuff)
>      (let* ([field* (syntax->list #'(field ...))]
>             [fields (map (lambda (f)
>                            (syntax-case f ()
>                              [(name default) #'name]
>                              [name f]))
>                          field*)]
>             [field-to-kw-param
>              (lambda (f)
>                `(,(string->keyword (symbol->string (syntax-e
> f))) ,f))] [field-with-default-to-kw-param
>              (lambda (f)
>                (define-values (name default) (apply values
> (syntax->list f))) (define name-as-keyword 
>                  (string->keyword (symbol->string (syntax-e name))))
>                `(,name-as-keyword (,name ,default)))]
>             [parameters 
>              (foldr (lambda (f r) 
>                       (append
>                        (syntax-case f ()
>                          [[name default]
> (field-with-default-to-kw-param f)] [name (field-to-kw-param f)])
>                        r))
>                     '[] field*)]
>             [name/kw 
>              (datum->syntax stx
>                             (string->symbol 
>                              (string-append (symbol->string (syntax-e
> #'name)) "/kw")))])
>        #`(begin
>            (struct name #,fields . stuff)
>            (define (#,name/kw #, at parameters)
>              (name #, at fields))))]))
> 
> ;;
> ----------------------------------------------------------------------------- ;;
> usage example (could be separate module) (struct/kw book (author
> title [details ""]) #:transparent)
> 
> (define book1 (book/kw #:title "The Client" #:author "John Grisham"))
> 
> (define book2 (book/kw #:title "The Client" #:author "John Grisham"
> #:details "???"))
> 
> (and (string=? (book-author book1) "John Grisham") 
>      (string=? (book-details book2) "???"))
> 
> 

Thanks a lot. That is really great. 

When I will study define-syntax stuff I will try to understand what you
did. So also thanks for your teaching.




-- 
Manfred