On Wed, 8 Dec 2010 10:25:47 -0500 Matthias Felleisen <matthias-1vnkwvzi4qavc3sceru...@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 _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
