[racket] Web-cells, anonymous functions, and garbage collection

I'm pretty sure code is collected in (regular) Racket. Small numbers aren't
collected, true, but they also don't really take any space. Big numbers are
collected, too. (I'm not sure about interned symbols.)

Robby


On Thu, Feb 14, 2013 at 7:26 AM, Jay McCarthy <jay.mccarthy at gmail.com>wrote:

> In the Web server language, all anonymous functions are turned into
> top-level structures masquerading as functions where the fields are
> their free variables. For instance:
>
> (define (f x) (lambda (y) (+ x y))
>
> becomes, roughly,
>
> (struct a-lambda42 (x)
>  #:property prop:procedure
>  (lambda (the-structure y)
>   (define x (a-lambda42-x the-structure))
>   (+ x y)))
> (define (f x) (make-a-lambda42 x))
>
> In your program, the expression
>
> (web-cell-shadow wc-anonymous (λ (x) 'more-distinctive-text))
>
> becomes
>
> (struct a-lambda43 () #:property prop:procedure (lambda (s x)
> 'more-distinctive-text))
> (web-cell-shadow wc-anonymous (make-a-lambda43))
>
> Notice that the anonymous function has no fields because there are no
> free variables. The distinctive text is not shown anywhere in the URL
> because it is part of the code, not part of the data, of the closure.
>
> If you change the program slightly to
>
> (web-cell-shadow wc-anonymous
>                  (let () (define y 'more-distinctive-text)
>                       (λ (x) y)))
>
> then y is a free variable, so when you look at the URL:
>
>
> http://localhost:8000/;((%22c%22%20.%20%220((3)%204%20(((lib%20%5C%22web-server%2Flang%2Fabort-resume.rkt%5C%22)%20.%20%5C%22lifted.6%5C%22)%20((lib%20%5C%22web-server%2Flang%2Fweb-cells.rkt%5C%22)%20.%20deserialize-info:frame-v0)%20(%23%5C%22%2Ftmp%2Ft.rkt%5C%22%20.%20%5C%22lifted.1683%5C%22)%20(2%20.%20%5C%22lifted.1991%5C%22))%200%20()%20()%20(0%20(1%20(h%20-%20()%20(lifted.1485-0%20.%20this-is-a-song-about-Japan)%20(lifted.1521-1%202%20(b!%20.%20more-distinctive-text))))%20(c%20(v!%20(3)%20%23f%20%23f))))%22))
>
> You see the distinctive text.
>
> As far as GC goes, code is not generally collected in Racket anyways.
> Any data that is an intrinsic part of the program text (such as
> literal symbols and numbers) will always be around in your program
> whether you use #lang web-server or not.
>
> Jay
>
>
> On Wed, Feb 13, 2013 at 8:34 PM, Galler <lzgaller at optonline.net> wrote:
> > Good evening,
> >
> > I've been investigating lang web-server, performance and garbage
> collection
> >
> > I have a question regarding anonymous functions and web cells
> >
> > Examples below require:
> >
> > #lang web-server
> > (require racket/serialize
> >          web-server/lang/web-cells)
> >
> >
> > Constants appear serializable by their value being explicitly written out
> >
> >
> > For example,
> >
> > (serialize 'this-is-a-song-about-japan)
> >
> > ;-> '((3) 0 () 0 () () this-is-a-song-about-japan)
> >
> >
> > Whereas anonymous functions appear serializable by the mechanism of being
> > "lifted" (i.e. reified) to a module level binding
> >
> > For example
> >
> > (serialize (λ (x) x))
> >
> > ;-> '((3) 1 (('anonymous-module . "lifted.1386")) 0 () () (0))
> >
> >
> > Moving the discussion to web-cells
> >
> > (define wc-constant (make-web-cell #f))
> >
> > (define wc-anonymous (make-web-cell #f))
> >
> >
> > (web-cell-shadow wc-constant 'this-is-a-song-about-Japan)
> >
> > (web-cell-shadow wc-anonymous (λ (x) x))
> >
> >  (printf " wc-constant: ~A\n wc-anonymous: ~A\n"
> >           (serialize wc-1)
> >           (serialize wc-2))
> >
> > returns
> >
> > wc-constant: ((3) 1 (((lib web-server/lang/web-cells.rkt) .
> > deserialize-info:primitive-wc-v0)) 0 () () (0 lifted.1534-0))
> >
> > wc-anonymous: ((3) 1 (((lib web-server/lang/web-cells.rkt) .
> > deserialize-info:primitive-wc-v0)) 0 () () (0 lifted.1570-1))
> >
> >
> >
> > While now both values are 'lifted' (i.e. reified), a close examination of
> > the URL-string sent back to a client after send/suspend/dispatch
> >
> > reveals that, with respect to the constant, lifted.1534 appears to be
> > explicitly represented, elsewhere in the string as an associative pair
> i.e.
> > (lifted.1534 this-is-a-song-about-Japan)
> >
> > whereas for the anonymous function, its not clear that its reference,
> > lifted.1570 is explicitly represented
> >
> > Which leads me to wonder about whether the reference to the anonymous
> > function, lifted.1570, is being maintained on the server,
> >
> > And if so, how is it being garbage collected, as its impossible to ever
> > determine that it is unreachable.
> >
> > I will also point out that prior to a year ago, anonymous functions
> failed
> > the serializability predicate. (serializable? anon-func) -> #f. I can
> pull
> > out the correspondence/background related to that if desirable.
> >
> > I apologize if I'm just misreading the data.
> >
> > Here's the complete example,with a stateless web server. Note that when
> you
> > inspect the url-string**for the datum this-is-a-song-about-Japan, its
> there,
> > but when looking for the datum more-distinctive-text, its not present in
> the
> > url-string.
> >
> > **right click on the link, then copy/paste, or click the 'click-me' link
> and
> > copy/paste the URL from the address bar)
> >
> > Thanks very much.
> >
> >
> > Thanks again.
> >
> >
> >
> > #lang web-server
> >
> > (require  web-server/servlet-env)
> >
> > (define wc-constant (make-web-cell #f))
> >
> > (define wc-anonymous (make-web-cell #f))
> >
> >
> > (web-cell-shadow wc-constant 'this-is-a-song-about-Japan)
> >
> > (web-cell-shadow wc-anonymous (λ (x) 'more-distinctive-text))
> >
> > (define (start request)
> >   (letrec ((response-generator (λ (make-url)
> >                               (response/xexpr `(html (head)
> >                                                      (body (a ((href
> > ,(format "~A" (make-url receive-request)))) "click me"))))))
> >            (receive-request (λ (request)
> >                               (response/xexpr `(html (head)
> >                                                      (body "Thank you. We
> > are done."))))))
> >     (send/suspend/dispatch response-generator)))
> >
> > (serve/servlet start
> >                #:stateless? #t
> >                #:launch-browser? #t
> >                #:connection-close? #t
> >                #:quit? #f
> >                #:listen-ip #f
> >                #:port 8000
> >                #:servlet-path "/")
> >
> >
> >
> > Thanks much.
> >
> > Zack
>
>
>
> --
> Jay McCarthy <jay at cs.byu.edu>
> Assistant Professor / Brigham Young University
> http://faculty.cs.byu.edu/~jay
>
> "The glory of God is Intelligence" - D&C 93
>
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users
>
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