[racket] structure question

On Wed, 8 Dec 2010 10:25:47 -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 
> 
> (define-syntax (struct/kw stx)
>   (syntax-case stx ()
>     [(_ name (field ...) . stuff)
>      (let* ([symbol-to-keyword-parameter
>              (lambda (s)
>                `(,(string->keyword (symbol->string (syntax-e
> s))) ,s))] [args 
>              (map symbol-to-keyword-parameter (syntax->list
> #'(field ...)))] [name/kw 
>              (datum->syntax stx
>                             (string->symbol 
>                              (string-append (symbol->string (syntax-e
> #'name)) "/kw")))])
>        #`(begin
>            (struct name (field ...) . stuff)
>            (define (#,name/kw #,@(foldl append '[] args))
>              (name field ...))))]))
> 
> ;;
> ----------------------------------------------------------------------------- ;;
> usage example (could be separate module) (struct/kw book (author
> title))
> 
> (define book1 (book/kw #:title "The Client" #:author "John Grisham" ))
> 
> (string=? (book-author book1) "John Grisham")
> 


Thanks very much. This looks really good. Would it be possible to define
a default value for certain members of a structure?

Perhaps like this:

(struct/kw book (author title [details (#:default-value "")]))
or this
(struct/kw book (author title [details #:default]) #:default-value "")



-- 
Manfred