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

That is also what I meant by code (well, I was speaking generally about all
bytecode).

The point I'm making is that this is not a limitation of our runtime
system, it is just because you are imagining there are still references to
that bytecode.

In the specific instance you mention below, the namespace holding onto that
module could become garbage. This happens in DrRacket a lot, for example.


On Thu, Feb 14, 2013 at 7:45 AM, Jay McCarthy <jay.mccarthy at gmail.com>wrote:

> When I say "code", I mean module top-level binding byte-code. There's
> no way for Racket to remove
>
> (define (f x) (lambda (y) (printf "Gigantic string: ~a" (+ x y))))
>
> at the top-level. The byte-code will always contain the gigantic
> string. If you ever call f and get back something, that closure could
> be collected, but not the source that generated it.
>
> Jay
>
> On Thu, Feb 14, 2013 at 6:37 AM, Robby Findler
> <robby at eecs.northwestern.edu> wrote:
> > 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
> >
> >
>
>
>
> --
> 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
>
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