2012/7/8 <jukka.tuomi...@finndesign.fi>:
>
> I came across this "mutable-define" by Eli (3+ years back)...
>
> ;; http://paste.lisp.org/display/67203
> ;; this is a PLT version, translate as necessary
>
> (define-syntax-rule (define-mutable name expr)
> (begin (define b (box expr))
> (define-syntax name
> (syntax-id-rules (set!)
> [(set! name new) (set-box! b new)]
> [(name . xs) ((unbox b) . xs)]
> [name (unbox b)]))))
>
> ;(define x 1)
> ;(define x 2)
>
>
> ...which seems beautifully concise for the job. The only problem is that
> it only seems to work in the interactions window in drracket. Is there a
> way to have it working inside a module? ... hopefully so that all the
> tweaks would take place inside the very module, rather than "required"
> from elsewhere (that would keep it an GP individual's property/"gene").
>
> This is obviously what Danny is trying to explain me - "...language
> variant of Racket where "define" acts like redefinition at the module
> toplevel..." - but once again, I seem to be reaching out things before
> understanding them.
Combining Dannys def and Eli's define-mutable I get this:
#lang racket
;; Returns true if stx is an identifier that's been lexically bound.
(define-for-syntax (lexically-bound? stx)
(let ([expanded (local-expand stx (syntax-local-context) #f)])
(not (and (identifier? expanded)
(eq? #f (identifier-binding expanded))))))
(define-syntax (defm stx)
(syntax-case stx ()
[(_ name expr)
(and (identifier? #'name) (lexically-bound? #'name))
(syntax/loc stx (set! name expr))]
[(_ name expr)
(identifier? #'name)
(syntax/loc stx
(begin
(define b (box expr))
(define-syntax name
(make-set!-transformer
(lambda (stx)
(syntax-case stx (set!)
[(set! name new) #'(set-box! b new)]
[(name . xs) #'((unbox b) . xs)]
[name #'(unbox b)]))))))]))
;;; Testing
(defm x 1)
x
(list (+ x 40))
(defm x 2)
x
(list (+ x 40))
(set! x 3)
x
(defm y 4)
y
(list (+ y 40))
(defm y 5)
y
(list (+ y 40))
(set! y 6)
y
; The output is
1
'(41)
2
'(42)
3
4
'(44)
5
'(45)
6
--
Jens Axel Søgaard
____________________
Racket Users list:
http://lists.racket-lang.org/users
