err... https://gist.github.com/1112122
On 2011-07-28, at 12:10 PM, Tom McNulty wrote: > Hello, > > I posted a question on stackoverflow this week: > http://stackoverflow.com/questions/6838115/racket-using-a-macro-across-modules, > with thanks to Eli, I was able to understand where I was going wrong. I > took his advice in using a struct-info alternative, and as I have limited > experience building macros I'm appealing for comments on my solution. > > The macro in question destructively increments a field in a structure. In > this version, the getter and setter are pulled from extract-struct-info. I > was hoping that I could locate the getter (and setter) with just the field > and structure type but that information doesn't seem to be possible. > > https://gist.github.com/1111993 > > Thanks, > > - Tom > > (for completeness, contents of the gist above) > > #lang racket > > (require (for-syntax racket/struct-info)) > (require rackunit) > > ;; (increment! a-struct type field [amount 1]) -> void > ;; increments a mutable field in a structure > (define-syntax (increment! stx) > (syntax-case stx () > [(_ s sn fn i) > (with-syntax ([(_ _ _ getters setters _) > (extract-struct-info (syntax-local-value #'sn))]) > (let ([seek (string->symbol > (format "~a-~a" (syntax-e #'sn) (syntax-e #'fn)))]) > (let iter ([gets (syntax->list #'getters)] [sets (syntax->list > #'setters)]) > (cond [(null? gets) > (raise-syntax-error #f "unknown field-name" stx)] > [(eq? (syntax-e (car gets)) seek) > (if (identifier? (car sets)) > (with-syntax ([set! (car sets)] [get (car gets)]) > #'(set! s (+ i (get s)))) > (raise-syntax-error #f (format "~s not mutable" > (syntax-e #'fn)) stx))] > [else > (iter (cdr gets) (cdr sets))]))))] > [(increment! s sn fn) #'(increment! s sn fn 1)])) > > (struct vault ([dollars #:mutable] > pounds > [euros #:mutable])) > > (define v (vault 0 50 20)) > (increment! v vault dollars 100) > (increment! v vault euros) > > (test-case "sums" > (check-equal? (vault-dollars v) 100) > (check-equal? (vault-euros v) 21)) > ;(test-exn "not mutable" exn:fail:syntax? (λ () (increment! v vault pounds))) > ;(test-exn "unknown name" exn:fail:syntax? (λ () (increment! v vault yen))) > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
