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rackety "more" tutorial

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Matthew Flatt 2010-04-28 15:26:41 -06:00
parent 1991e9328f
commit 43cb91e6ee
11 changed files with 168 additions and 163 deletions
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309
collects/scribblings/more/more.scrbl
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@ -2,13 +2,13 @@
@(require scribble/manual
scribble/urls
scribble/eval
"../quick/keep.ss"
"../quick/keep.rkt"
(for-label scheme
scheme/enter
racket/enter
readline
net/url
xml
scheme/control))
racket/control))
@(define quick @other-manual['(lib "quick.scrbl" "scribblings/quick")])
@(define guide @other-manual['(lib "guide.scrbl" "scribblings/guide")])
@ -16,7 +16,7 @@
@(define more-eval (make-base-eval))
@(interaction-eval #:eval more-eval
(define (show-load re?)
(fprintf (current-error-port) " [~aloading serve.ss]\n" (if re? "re-" ""))))
(fprintf (current-error-port) " [~aloading serve.rkt]\n" (if re? "re-" ""))))
@(interaction-eval #:eval more-eval
(define (serve n) void))
@(interaction-eval #:eval more-eval
@ -39,13 +39,13 @@
" in plain text: "
(link file "step " which) ".")))
@title{@bold{More}: Systems Programming with PLT Scheme}
@title{@bold{More}: Systems Programming with Racket}
@author["Matthew Flatt"]
In contrast to the impression that @|quick| may give, PLT Scheme is
In contrast to the impression that @|quick| may give, Racket is
not just another pretty face. Underneath the graphical facade of
DrScheme lies a sophisticated toolbox for managing threads and
DrRacket lies a sophisticated toolbox for managing threads and
processes, which is the subject of this tutorial.
Specifically, we show how to build a secure, multi-threaded,
@ -54,39 +54,39 @@ the language than in @|quick|, and we expect you to click on syntax or
function names that you don't recognize (which will take you to the
relevant documentation). Beware that the last couple of sections
present material that is normally considered difficult. If you're
still new to Scheme and have relatively little programming experience,
still new to Racket and have relatively little programming experience,
you may want to skip to @|guide|.
To get into the spirit of this tutorial, we suggest that you set
DrScheme aside for a moment, and switch to raw @exec{mzscheme} in a
DrRacket aside for a moment, and switch to raw @exec{racket} in a
terminal. You'll also need a text editor, such as @exec{emacs} or
@exec{vi}. Finally, you'll need a web client, perhaps @exec{lynx} or
@exec{firefox}.
@margin-note{Of course, if you're already spoiled, you can keep using
DrScheme.}
DrRacket.}
@; ----------------------------------------------------------------------
@section{Ready...}
@link[url:download-drscheme]{Download PLT Scheme}, install, and then
start @exec{mzscheme} with no command-line arguments:
@link[url:download-drracket]{Download Racket}, install, and then
start @exec{racket} with no command-line arguments:
@verbatim[#:indent 2]{
$ mzscheme
Welcome to MzScheme
$ racket
Welcome to Racket
>
}
If you're using a plain terminal, if you have GNU Readline installed
on your system, and if you'd like Readline support in @exec{mzscheme},
then evaluate @scheme[(require readline)]. If you also evaluate
@scheme[(install-readline!)], then your @filepath{~/.mzschemerc} is
updated to load Readline whenever you start @exec{mzscheme} for
on your system, and if you'd like Readline support in @exec{racket},
then evaluate @racket[(require readline)]. If you also evaluate
@racket[(install-readline!)], then your @filepath{~/.racketrc} is
updated to load Readline whenever you start @exec{racket} for
interactive evaluation.
@margin-note{Unfortunately, for legal reasons related to GPL vs. LGPL,
@exec{mzscheme} cannot provide Readline automatically.}
@exec{racket} cannot provide Readline automatically.}
@interaction[
(eval:alts (require readline) (void))
@ -96,11 +96,11 @@ interactive evaluation.
@; ----------------------------------------------------------------------
@section{Set...}
In the same directory where you started @exec{mzscheme}, create a text
file @filepath{serve.ss}, and start it like this:
In the same directory where you started @exec{racket}, create a text
file @filepath{serve.rkt}, and start it like this:
@schememod[
scheme
@racketmod[
racket
(define (go)
'yep-it-works)
@ -111,35 +111,35 @@ scheme
@; ----------------------------------------------------------------------
@section{Go!}
Back in @exec{mzscheme}, try loading the file and running @scheme[go]:
Back in @exec{racket}, try loading the file and running @racket[go]:
@interaction[
#:eval more-eval
(eval:alts (enter! "serve.ss") (show-load #f))
(eval:alts (enter! "serve.rkt") (show-load #f))
(eval:alts (go) 'yep-it-works)
]
Try modifying @filepath{serve.ss}, and then run @scheme[(enter!
"serve.ss")] again to re-load the module, and then check your changes.
Try modifying @filepath{serve.rkt}, and then run @racket[(enter!
"serve.rkt")] again to re-load the module, and then check your changes.
@; ----------------------------------------------------------------------
@section{``Hello World'' Server}
We'll implement the web server through a @scheme[serve] function that
We'll implement the web server through a @racket[serve] function that
takes a IP port number for client connections:
@schemeblock[
@racketblock[
(define (serve port-no)
...)
]
The server accepts TCP connections through a @defterm{listener}, which
we create with @scheme[tcp-listen]. To make interactive development
easier, we supply @scheme[#t] as the third argument to
@scheme[tcp-listen], which lets us re-use the port number without
we create with @racket[tcp-listen]. To make interactive development
easier, we supply @racket[#t] as the third argument to
@racket[tcp-listen], which lets us re-use the port number without
waiting on TCP timeouts.
@schemeblock[
@racketblock[
(define (serve port-no)
(define listener (tcp-listen port-no 5 #t))
...)
@ -147,7 +147,7 @@ waiting on TCP timeouts.
The server must loop to accept connections from the listener:
@schemeblock[
@racketblock[
(define (serve port-no)
(define listener (tcp-listen port-no 5 #t))
(define (loop)
@ -156,11 +156,11 @@ The server must loop to accept connections from the listener:
(loop))
]
Our @scheme[accept-and-handle] function accepts a connection using
@scheme[tcp-accept], which returns two values: a stream for input from
Our @racket[accept-and-handle] function accepts a connection using
@racket[tcp-accept], which returns two values: a stream for input from
the client, and a stream for output to the client.
@schemeblock[
@racketblock[
(define (accept-and-handle listener)
(define-values (in out) (tcp-accept listener))
(handle in out)
@ -171,7 +171,7 @@ the client, and a stream for output to the client.
To handle a connection, for now, we'll read and discard the request
header, and then write a ``Hello, world!'' web page as the result:
@schemeblock[
@racketblock[
(define (handle in out)
(code:comment @#,t{Discard the request header (up to blank line):})
(regexp-match #rx"(\r\n|^)\r\n" in)
@ -181,23 +181,23 @@ header, and then write a ``Hello, world!'' web page as the result:
(display "<html><body>Hello, world!</body></html>" out))
]
Note that @scheme[regexp-match] operates directly on the input stream,
Note that @racket[regexp-match] operates directly on the input stream,
which is easier than bothering with individual lines.
@whole-prog["1"]
Copy the above three definitions---@scheme[serve],
@scheme[accept-and-handle], and @scheme[handle]---into
@filepath{serve.ss} and re-load:
Copy the above three definitions---@racket[serve],
@racket[accept-and-handle], and @racket[handle]---into
@filepath{serve.rkt} and re-load:
@interaction[
#:eval more-eval
(eval:alts (enter! "serve.ss") (show-load #t))
(eval:alts (enter! "serve.rkt") (show-load #t))
(eval:alts (serve 8080) (void))
]
Now point your browser to @tt{http://localhost:8080} (assuming that
you used @scheme[8080] as the port number, and that the browser is
you used @racket[8080] as the port number, and that the browser is
running on the same machine) to receive a friendly greeting from your
web server.
@ -205,10 +205,10 @@ web server.
@section{Server Thread}
Before we can make the web server respond in more interesting ways, we
need to get a Scheme prompt back. Typing Ctl-C in your terminal window
need to get a Racket prompt back. Typing Ctl-C in your terminal window
interrupts the server loop:
@margin-note{In DrScheme, instead of typing Ctl-C, click the
@margin-note{In DrRacket, instead of typing Ctl-C, click the
@onscreen{Stop} button once.}
@interaction[
@ -225,15 +225,15 @@ number:
(eval:alts (serve 8080) (show-fail 8080))
]
The problem is that the listener that we created with @scheme[serve]
The problem is that the listener that we created with @racket[serve]
is still listening on the original port number.
To avoid this problem, let's put the listener loop in its own thread,
and have @scheme[serve] return immediately. Furthermore, we'll have
@scheme[serve] return a function that can be used to shut down the
and have @racket[serve] return immediately. Furthermore, we'll have
@racket[serve] return a function that can be used to shut down the
server thread and TCP listener:
@schemeblock[
@racketblock[
(define (serve port-no)
(define listener (tcp-listen port-no 5 #t))
(define (loop)
@ -251,7 +251,7 @@ Try the new one:
@interaction[
#:eval more-eval
(eval:alts (enter! "serve.ss") (show-load #t))
(eval:alts (enter! "serve.rkt") (show-load #t))
(define stop (serve 8081))
]
@ -274,7 +274,7 @@ as you like:
In the same way that we put the main server loop into a background
thread, we can put each individual connection into its own thread:
@schemeblock[
@racketblock[
(define (accept-and-handle listener)
(define-values (in out) (tcp-accept listener))
(thread
@ -288,8 +288,8 @@ thread, we can put each individual connection into its own thread:
With this change, our server can now handle multiple threads at
once. The handler is so fast that this fact will be difficult to
detect, however, so try inserting @scheme[(sleep (random 10))] before
the @scheme[handle] call above. If you make multiple connections from
detect, however, so try inserting @racket[(sleep (random 10))] before
the @racket[handle] call above. If you make multiple connections from
the web browser at roughly the same time, some will return soon, and
some will take up to 10 seconds. The random delays will be independent
of the order in which you started the connections.
@ -304,10 +304,10 @@ like to implement a timeout for each connection thread.
One way to implement the timeout is to create a second thread that
waits for 10 seconds, and then kills the thread that calls
@scheme[handle]. Threads are lightweight enough in Scheme that this
@racket[handle]. Threads are lightweight enough in Racket that this
watcher-thread strategy works well:
@schemeblock[
@racketblock[
(define (accept-and-handle listener)
(define-values (in out) (tcp-accept listener))
(define t (thread
@ -322,22 +322,25 @@ watcher-thread strategy works well:
]
This first attempt isn't quite right, because when the thread is
killed, its @scheme[in] and @scheme[out] streams remain open. We
killed, its @racket[in] and @racket[out] streams remain open. We
could add code to the watcher thread to close the streams as well as
kill the thread, but Scheme offers a more general shutdown mechanism:
kill the thread, but Racket offers a more general shutdown mechanism:
@defterm{custodians}. A custodian is a kind of container for all
resources other than memory, and it supports a
@scheme[custodian-shutdown-all] operation that terminates and closes
@racket[custodian-shutdown-all] operation that terminates and closes
all resources within the container, whether they're threads, streams,
or other kinds of limited resources.
Whenever a thread or stream is created, it is placed into the current
custodian as determined by the @scheme[current-custodian]
custodian as determined by the @racket[current-custodian]
parameter. To place everything about a connection into a custodian, we
@scheme[parameterize] all the resource creations to go into a new
@racket[parameterize] all the resource creations to go into a new
custodian:
@schemeblock[
@margin-note{See @secref[#:doc '(lib "scribblings/guide/guide.scrbl") "parameterize"]
for an introduction to parameters.}
@racketblock[
(define (accept-and-handle listener)
(define cust (make-custodian))
(parameterize ([current-custodian cust])
@ -352,16 +355,16 @@ custodian:
(custodian-shutdown-all cust))))
]
With this implementation, @scheme[in], @scheme[out], and the thread
that calls @scheme[handle] all belong to @scheme[cust]. In addition,
if we later change @scheme[handle] so that it, say, opens a file, then
the file handles will also belong to @scheme[cust], so they will be
reliably closed when @scheme[cust] is shut down.
With this implementation, @racket[in], @racket[out], and the thread
that calls @racket[handle] all belong to @racket[cust]. In addition,
if we later change @racket[handle] so that it, say, opens a file, then
the file handles will also belong to @racket[cust], so they will be
reliably closed when @racket[cust] is shut down.
In fact, it's a good idea to change @scheme[serve] so that it uses a
In fact, it's a good idea to change @racket[serve] so that it uses a
custodian, too:
@schemeblock[
@racketblock[
(define (serve port-no)
(define main-cust (make-custodian))
(parameterize ([current-custodian main-cust])
@ -374,7 +377,7 @@ custodian, too:
(custodian-shutdown-all main-cust)))
]
That way, the @scheme[main-cust] created in @scheme[serve] not only
That way, the @racket[main-cust] created in @racket[serve] not only
owns the TCP listener and the main server thread, it also owns every
custodian created for a connection. Consequently, the revised shutdown
procedure for the server immediately terminates all active connections,
@ -382,17 +385,17 @@ in addition to the main server loop.
@whole-prog["4"]
After updating the @scheme[serve] and @scheme[accept-and-handle]
After updating the @racket[serve] and @racket[accept-and-handle]
functions as above, here's how you can simulate a malicious client:
@interaction[
#:eval more-eval
(eval:alts (enter! "serve.ss") (show-load #t))
(eval:alts (enter! "serve.rkt") (show-load #t))
(define stop (serve 8081))
(eval:alts (define-values (cin cout) (tcp-connect "localhost" 8081)) (void))
]
Now wait 10 seconds. If you try reading from @scheme[cin], which is
Now wait 10 seconds. If you try reading from @racket[cin], which is
the stream that sends data from the server back to the client, you'll
find that the server has shut down the connection:
@ -422,12 +425,12 @@ URLs.
To parse the incoming URL and to more easily format HTML output, we'll
require two extra libraries:
@schemeblock[
@racketblock[
(require xml net/url)
]
The @schememodname[xml] library gives us @scheme[xexpr->string], which
takes a Scheme value that looks like HTML and turns it into actual
The @racketmodname[xml] library gives us @racket[xexpr->string], which
takes a Racket value that looks like HTML and turns it into actual
HTML:
@interaction[
@ -435,11 +438,11 @@ HTML:
(xexpr->string '(html (head (title "Hello")) (body "Hi!")))
]
We'll assume that our new @scheme[dispatch] function (to be written)
We'll assume that our new @racket[dispatch] function (to be written)
takes a requested URL and produces a result value suitable to use with
@scheme[xexpr->string] to send back to the client:
@racket[xexpr->string] to send back to the client:
@schemeblock[
@racketblock[
(define (handle in out)
(define req
(code:comment @#,t{Match the first line to extract the request:})
@ -456,8 +459,8 @@ takes a requested URL and produces a result value suitable to use with
(display (xexpr->string xexpr) out))))
]
The @schememodname[net/url] library gives us @scheme[string->url],
@scheme[url-path], @scheme[path/param-path], and @scheme[url-query]
The @racketmodname[net/url] library gives us @racket[string->url],
@racket[url-path], @racket[path/param-path], and @racket[url-query]
for getting from a string to parts of the URL that it represents:
@interaction[
@ -468,11 +471,11 @@ for getting from a string to parts of the URL that it represents:
(url-query u)
]
We use these pieces to implement @scheme[dispatch]. The
@scheme[dispatch] function consults a hash table that maps an initial
path element, like @scheme["foo"], to a handler function:
We use these pieces to implement @racket[dispatch]. The
@racket[dispatch] function consults a hash table that maps an initial
path element, like @racket["foo"], to a handler function:
@schemeblock[
@racketblock[
(define (dispatch str-path)
(code:comment @#,t{Parse the request as a URL:})
(define url (string->url str-path))
@ -493,17 +496,17 @@ path element, like @scheme["foo"], to a handler function:
(define dispatch-table (make-hash))
]
With the new @scheme[require] import and new @scheme[handle],
@scheme[dispatch], and @scheme[dispatch-table] definitions, our
With the new @racket[require] import and new @racket[handle],
@racket[dispatch], and @racket[dispatch-table] definitions, our
``Hello World!'' server has turned into an error server. You don't have
to stop the server to try it out. After modifying @filepath{serve.ss}
with the new pieces, evaluate @scheme[(enter! "serve.ss")] and then
to stop the server to try it out. After modifying @filepath{serve.rkt}
with the new pieces, evaluate @racket[(enter! "serve.rkt")] and then
try again to connect to the server. The web browser should show an
``Unknown page'' error in red.
We can register a handler for the @scheme["hello"] path like this:
We can register a handler for the @racket["hello"] path like this:
@schemeblock[
@racketblock[
(hash-set! dispatch-table "hello"
(lambda (query)
`(html (body "Hello, World!"))))
@ -511,25 +514,25 @@ We can register a handler for the @scheme["hello"] path like this:
@whole-prog["5"]
After adding these lines and evaluating @scheme[(enter! "serve.ss")],
After adding these lines and evaluating @racket[(enter! "serve.rkt")],
opening @tt{http://localhost:8081/hello} should produce the old
greeting.
@; ----------------------------------------------------------------------
@section{Servlets and Sessions}
Using the @scheme[query] argument that is passed to a handler by
@scheme[dispatch], a handler can respond to values that a user
Using the @racket[query] argument that is passed to a handler by
@racket[dispatch], a handler can respond to values that a user
supplies through a form.
The following helper function constructs an HTML form. The
@scheme[label] argument is a string to show the user. The
@scheme[next-url] argument is a destination for the form results. The
@scheme[hidden] argument is a value to propagate through the form as a
hidden field. When the user responds, the @scheme["number"] field in
@racket[label] argument is a string to show the user. The
@racket[next-url] argument is a destination for the form results. The
@racket[hidden] argument is a value to propagate through the form as a
hidden field. When the user responds, the @racket["number"] field in
the form holds the user's value:
@schemeblock[
@racketblock[
(define (build-request-page label next-url hidden)
`(html
(head (title "Enter a Number to Add"))
@ -547,7 +550,10 @@ the form holds the user's value:
Using this helper function, we can create a servlet that generates as
many ``hello''s as a user wants:
@schemeblock[
@margin-note{See @secref[#:doc '(lib "scribblings/guide/guide.scrbl") "for"]
for an introduction to forms like @racket[for/list].}
@racketblock[
(define (many query)
(build-request-page "Number of greetings:" "/reply" ""))
@ -563,23 +569,23 @@ many ``hello''s as a user wants:
@whole-prog["6"]
As usual, once you have added these to your program, update with
@scheme[(enter! "serve.ss")], and then visit
@racket[(enter! "serve.rkt")], and then visit
@tt{http://localhost:8081/many}. Provide a number, and you'll receive
a new page with that many ``hello''s.
@; ----------------------------------------------------------------------
@section{Limiting Memory Use}
With our latest @scheme["many"] servlet, we seem to have a new
With our latest @racket["many"] servlet, we seem to have a new
problem: a malicious client could request so many ``hello''s that the
server runs out of memory. Actually, a malicious client could also
supply an HTTP request whose first line is arbitrarily long.
The solution to this class of problems is to limit the memory use of a
connection. Inside @scheme[accept-and-handle], after the definition of
@scheme[cust], add the line
connection. Inside @racket[accept-and-handle], after the definition of
@racket[cust], add the line
@schemeblock[(custodian-limit-memory cust (* 50 1024 1024))]
@racketblock[(custodian-limit-memory cust (* 50 1024 1024))]
@whole-prog["7"]
@ -591,15 +597,15 @@ for each other's memory use, so one misbehaving connection will not
interfere with a different one.
So, with the new line above, and assuming that you have a couple of
hundred megabytes available for the @exec{mzscheme} process to use,
hundred megabytes available for the @exec{racket} process to use,
you shouldn't be able to crash the web server by requesting a
ridiculously large number of ``hello''s.
Given the @scheme["many"] example, it's a small step to a web server
that accepts arbitrary Scheme code to execute on the server. In that
Given the @racket["many"] example, it's a small step to a web server
that accepts arbitrary Racket code to execute on the server. In that
case, there are many additional security issues besides limiting
processor time and memory consumption. The
@schememodname[scheme/sandbox] library provides support to managing
@racketmodname[racket/sandbox] library provides support to managing
all those other issues.
@; ----------------------------------------------------------------------
@ -607,9 +613,9 @@ all those other issues.
As a systems example, the problem of implementing a web server exposes
many system and security issues where a programming language can
help. The web-server example also leads to a classic, advanced Scheme
help. The web-server example also leads to a classic, advanced Racket
topic: @defterm{continuations}. In fact, this facet of a web server
needs @defterm{delimited continuations}, which PLT Scheme provides.
needs @defterm{delimited continuations}, which Racket provides.
The problem solved by continuations is related to servlet sessions and
user input, where a computation spans multiple client connections
@ -619,11 +625,11 @@ a mixture of techniques (e.g., to take advantage of the browser's
``back'' button).
As the multi-connection computation becomes more complex, propagating
arguments through @scheme[query] becomes increasingly tedious. For
arguments through @racket[query] becomes increasingly tedious. For
example, we can implement a servlet that takes two numbers to add by
using the hidden field in the form to remember the first number:
@schemeblock[
@racketblock[
(define (sum query)
(build-request-page "First number:" "/one" ""))
@ -647,7 +653,7 @@ using the hidden field in the form to remember the first number:
While the above works, we would much rather write such computations in
a direct style:
@schemeblock[
@racketblock[
(define (sum2 query)
(define m (get-number "First number:"))
(define n (get-number "Second number:"))
@ -656,32 +662,32 @@ a direct style:
(hash-set! dispatch-table "sum2" sum2)
]
The problem is that @scheme[get-number] needs to send an HTML response
The problem is that @racket[get-number] needs to send an HTML response
back for the current connection, and then it must obtain a response
through a new connection. That is, somehow it needs to convert the
page generated by @scheme[build-request-page] into a @scheme[query]
page generated by @racket[build-request-page] into a @racket[query]
result:
@schemeblock[
@racketblock[
(define (get-number label)
(define query
... (build-request-page label ...) ...)
(number->string (cdr (assq 'number query))))
]
Continuations let us implement a @scheme[send/suspend] operation that
performs exactly that operation. The @scheme[send/suspend] procedure
Continuations let us implement a @racket[send/suspend] operation that
performs exactly that operation. The @racket[send/suspend] procedure
generates a URL that represents the current connection's computation,
capturing it as a continuation. It passes the generated URL to a
procedure that creates the query page; this query page is used as the
result of the current connection, and the surrounding computation
(i.e., the continuation) is aborted. Finally, @scheme[send/suspend]
(i.e., the continuation) is aborted. Finally, @racket[send/suspend]
arranges for a request to the generated URL (in a new connection) to
restore the aborted computation.
Thus, @scheme[get-number] is implemented as follows:
Thus, @racket[get-number] is implemented as follows:
@schemeblock[
@racketblock[
(define (get-number label)
(define query
(code:comment @#,t{Generate a URL for the current computation:})
@ -695,40 +701,39 @@ Thus, @scheme[get-number] is implemented as follows:
(string->number (cdr (assq 'number query))))
]
We still have to implement @scheme[send/suspend]. Plain Scheme's
@scheme[call/cc] is not quite enough, so we import a library of
control operators:
We still have to implement @racket[send/suspend]. For that task, we
import a library of control operators:
@schemeblock[(require scheme/control)]
@racketblock[(require racket/control)]
Specifically, we need @scheme[prompt] and @scheme[abort] from
@schememodname[scheme/control]. We use @scheme[prompt] to mark the
Specifically, we need @racket[prompt] and @racket[abort] from
@racketmodname[racket/control]. We use @racket[prompt] to mark the
place where a servlet is started, so that we can abort a computation
to that point. Change @scheme[handle] by wrapping an @scheme[prompt]
around the call to @scheme[dispatch]:
to that point. Change @racket[handle] by wrapping an @racket[prompt]
around the call to @racket[dispatch]:
@schemeblock[
@racketblock[
(define (handle in out)
....
(let ([xexpr (prompt (dispatch (list-ref req 1)))])
....))
]
Now, we can implement @scheme[send/suspend]. We use @scheme[call/cc]
in the guise of @scheme[let/cc], which captures the current
computation up to an enclosing @scheme[prompt] and binds that
computation to an identifier---@scheme[k], in this case:
Now, we can implement @racket[send/suspend]. We use @racket[call/cc]
in the guise of @racket[let/cc], which captures the current
computation up to an enclosing @racket[prompt] and binds that
computation to an identifier---@racket[k], in this case:
@schemeblock[
@racketblock[
(define (send/suspend mk-page)
(let/cc k
...))
]
Next, we generate a new dispatch tag, and we record the mapping from
the tag to @scheme[k]:
the tag to @racket[k]:
@schemeblock[
@racketblock[
(define (send/suspend mk-page)
(let/cc k
(define tag (format "k~a" (current-inexact-milliseconds)))
@ -737,10 +742,10 @@ the tag to @scheme[k]:
]
Finally, we abort the current computation, supplying instead the page
that is built by applying the given @scheme[mk-page] to a URL for the
that is built by applying the given @racket[mk-page] to a URL for the
generated tag:
@schemeblock[
@racketblock[
(define (send/suspend mk-page)
(let/cc k
(define tag (format "k~a" (current-inexact-milliseconds)))
@ -751,21 +756,21 @@ generated tag:
When the user submits the form, the handler associated with the form's
URL is the old computation, stored as a continuation in the dispatch
table. Calling the continuation (like a function) restores the old
computation, passing the @scheme[query] argument back to that
computation, passing the @racket[query] argument back to that
computation.
@whole-prog["9" #t]
In summary, the new pieces are: @scheme[(require scheme/control)],
adding @scheme[prompt] inside @scheme[handle], the definitions of
@scheme[send/suspend], @scheme[get-number], and @scheme[sum2], and
@scheme[(hash-set! dispatch-table "sum2" sum2)]. Once you have
In summary, the new pieces are: @racket[(require racket/control)],
adding @racket[prompt] inside @racket[handle], the definitions of
@racket[send/suspend], @racket[get-number], and @racket[sum2], and
@racket[(hash-set! dispatch-table "sum2" sum2)]. Once you have
the server updated, visit @tt{http://localhost:8081/sum2}.
@; ----------------------------------------------------------------------
@section{Where to Go From Here}
The PLT Scheme distribution includes a production-quality web server
The Racket distribution includes a production-quality web server
that addresses all of the design points mentioned here and more.
To learn more, see the tutorial @other-manual['(lib
"web-server/scribblings/tutorial/continue.scrbl")], the
@ -773,8 +778,8 @@ documentation @other-manual['(lib
"web-server/scribblings/web-server.scrbl")], or the research paper
@cite["Krishnamurthi07"].
Otherwise, if you arrived here as part of an introduction to PLT
Scheme, then your next stop is probably @|guide|.
Otherwise, if you arrived here as part of an introduction to
Racket, then your next stop is probably @|guide|.
If the topics covered here are the kind that interest you, see also
@secref["concurrency" #:doc '(lib
@ -783,10 +788,10 @@ If the topics covered here are the kind that interest you, see also
"scribblings/reference/reference.scrbl")].
Some of this material is based on relatively recent research, and more
information can be found in papers written by the authors of PLT
Scheme, including papers on MrEd @cite["Flatt99"], memory accounting
@cite["Wick04"], kill-safe abstractions @cite["Flatt04"], and
delimited continuations @cite["Flatt07"].
information can be found in papers written by the authors of Racket,
including papers on GRacket (formerly ``MrEd'') @cite["Flatt99"],
memory accounting @cite["Wick04"], kill-safe abstractions
@cite["Flatt04"], and delimited continuations @cite["Flatt07"].
@; ----------------------------------------------------------------------

2
collects/scribblings/more/step0.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
(define (go)
'yep-it-works)

2
collects/scribblings/more/step1.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
(define (serve port-no)
(define listener (tcp-listen port-no 5 #t))

2
collects/scribblings/more/step2.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
;; The `serve' function is revised to run the loop
;; in a thread, and it returns a function to shut down

2
collects/scribblings/more/step3.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
;; Only `accept-and-handle' changes, moving the
;; handle work into a thread.

2
collects/scribblings/more/step4.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
;; Both `server' and `accept-and-handle' change
;; to use a custodian.

2
collects/scribblings/more/step5.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
;; New imports:
(require xml net/url)

2
collects/scribblings/more/step6.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
;; For changes, skip down to `build-request-page',
;; after the line.

2
collects/scribblings/more/step7.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
;; There's only one change. It's in `accept-and-handle',
;; and it's marked with "<<<".

2
collects/scribblings/more/step8.txt
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@ -1,4 +1,4 @@
#lang scheme
#lang racket
;; No changes to old code --- just three new
;; functions and registrations at the end.

4
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@ -1,10 +1,10 @@
#lang scheme
#lang racket
;; See "<<<" for two small changes, then jump down
;; to `send/suspend'.
(require xml net/url
scheme/control) ;; <<< new import
racket/control) ;; <<< new import
(define (serve port-no)
(define main-cust (make-custodian))