jukka.tuominen | 8 Jul 21:04 2012
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Re: Redefinition of initial bindings/ Resessive genes in GP


Not that I would understand any of this, but it works perfectly!

Thank you so much Jens, Danny and Eli!

br, jukka

> 2012/7/8  <jukka.tuominen <at> 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 S√łgaard
>

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