sunet/doc/latex/surflethowto.tex

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%%title{Howto write \surflets}
%%author{Andreas Bernauer}
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\section{Howto}
\label{sec:surflethowto}

This howto gives a short introduction in how to write a \surflet.  It
is concentrated on the practical side rather on describing the
\surflet API in detail to give you instant succes in running your own
surflets.  See section \ref{sec:surflet-api} for the (technical) API
description. 

\subsection{Introduction}
%\marginpar{\surflets are pieces of code for web site scripting.}
For those who don't know it already, \surflets are pieces of code that
can be executed interactively through a website.  There is a \surflet
handler who administrates their execution and suspension.  The
\surflet handler is part of the SUnet webserver.  \surflets ease the
implementation of web applications in two ways, compared to other
server-side scripting tools like Java\texttrademark Servlets or
Microsoft\textregistered Active Server Pages or PHP:

\begin{enumerate}
\item  \surflets have an automatic program flow control like any
other usual program (but unlike usual web programs), \ie the web
designer doesn't have to care about session management at all. The
sequence of the web pages result from their appearance in the program
like the print statements in any other usual program.

\item  \surflets come along with a library for robust user
interaction.  \surflets represent interaction elements of the web page
like text input fields or dropdown lists in the \surflet program by
specific objects.  A web designer can plug in these objects into a
website and use them to read out the user input.
\end{enumerate}

The following sections probably assume that you have basic knowledge
of the SUnet webserver and scsh.  The environment variable
\typew{\$sunet} refers to the top level directory of your sunet
installation.  On my system this is \name{/home/andreas/sw/sunet}.

\subsection{How to run the SUnet webserver that handles \surflets}

The following sections will show pieces of \surflet code you might
want to try out.  Therefore you need the SUnet webserver running with
the ability to serve \surflets.  This section tells you how to do it.

\paragraph{Obtaining necessary packages} You need Oleg's SSAX package
(for scsh), to be able to use surflets:

\begin{itemize}
  \item Download Oleg's SSAX package from
  \url{http://prdownloads.sf.net/ssax/ssax-sr5rs-plt200-4.9.tar.gz?download}.

  \item Download the SSAX package kit from
     \url{http://lamp.epfl.ch/~schinz/scsh_packages}.  

  \item Uncompress and untar both tarballs in the same directory.
  This will create a directory called \name{SSAX}, to which I will
  refer to as \typew{\$SSAX}.  The package kit will add a file
  \name{pkg-def.scm} to the \name{SSAX} directory.

  \item Install SSAX as a scsh package by issuing the command
  \typew{scsh-install-pkg --prefix /path/to/your/package/root} in the
  \typew{\$SSAX} directory.  If you don't yet have the packaging
  utility of Michel Schinz, you can obtain it from
  \url{http://lamp.epfl.ch/~schinz/scsh_packages}.

  If you don't want to install SSAX with the packaging utility, you
  can adjust the scripts to directly load the SSAX package definitions
  from \type{\$SSAX/lib/packages.scm}.  Note that the original file
  has a typo which you can correct with

  \begin{alltt}
    cd \$SSAX
    patch -p1 < \$sunet/httpd/surflets/SSAX-goodhtml-patch
  \end{alltt}


%  \item Unfortunately, this SSAX distribution (plt200-4.9) has a typo in
%  the package definition for scsh.  Apply the patch that comes with
%  the \surflets distribution to get rid of this typo:
%    \begin{itemize}
%      \item \typew{cd \$SSAX}
%      \item \typew{patch -p1 < \$sunet/httpd/surflets/SSAX-goodhtml-patch}
%    \end{itemize}
\end{itemize}

\paragraph{Starting the SUrflet server} 
 You can start the SUnet webserver along with the
\surflet-handler now.  The SUnet distribution comes with a script that
does this for you:

\begin{alltt}
\$ /path/to/your/package/root/sunet/web-server/start-surflet-server
\end{alltt}

Please be patient, scsh has to load a lot of libraries.  If the
loading succeeds you will see something like this:

\begin{alltt}
[andreas@hgt web-server]\$ ./start-surflet-server
Loading...
reading options: ()
Going to run SUrflet server with:
 htdocs-dir:    /home/andreas/bin/lib/scsh/0.6/sunet-2.1/web-server/root/htdocs   
 surflet-dir:   /home/andreas/bin/lib/scsh/0.6/sunet-2.1/web-server/root/surflets 
 images-dir:    /home/andreas/bin/lib/scsh/0.6/sunet-2.1/web-server/root/img
 port:          8080
 log-file-name: /tmp/httpd.log
 a maximum of 5 simultaneous requests, syslogging activated, 
 and home-dir-handler (public_html) activated.

 NOTE: This is the SUrflet server. It does not support cgi.
\end{alltt}

This means the server is up and running.  Try to connect to
\url{http://localhost:8080} with your browser and you will see the
welcome page of the SUnet server.  There's a link to the
\surflets homepage.  You can also already try out some of the
\surflets that come with the distribution.

You will probably notice a long response time the first time you load
the first \surflet.  This is because the server has to load the
\surflet libraries.  The server handles further requests to \surflets
faster.

If the port the \surflet server tries to use is occupied, you will see
an error message similar to this one:

\begin{alltt}
Error: 98
       "Address already in use"
       #{Procedure 11701 (\%bind in scsh-level-0)}
       4
       2
       (0 . 8080)
\end{alltt}

In this case, pass another port number to the script, \eg 8000:
\codex{start-surflet-server -p 8000}

The \typew{--help} option will show you more parameters that you can
adjust, but you won't need them for this howto.


\subsection{How to send web pages}

This section will discuss some of the various ways in which you can
send a web page to a browser that contacted your \surflet.

\subsubsection{My first \surflet}
\label{sec:first-surflet}

Traditionally,  your first program in any programming language prints
something like ``Hello, World!''.  We follow this tradition:

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        scheme-with-scsh)
  (begin
    (define (main req)
      (send-html/finish
       '(html (body (h1 "Hello, world!")))))
    ))
\end{listing}

You can either save a file with that content in the \surflets
directory the server mentioned at startup or you can use the file
\name{howto/hello.scm} that comes along with the \surflets
distribution and which is located in the server's standard \surflets
directory.  Let's go through the small script step by step:

\begin{alltt}
(define-structure surflet surflet-interface
\end{alltt}

This defines a module named \name{surflet} which implements the
interface \name{surflet-interface}.  \name{surflet-interface} just
states that the module exports a function named \name{main} to which
we will come shortly.  For those of you who know about the scsh module
system: Yes, \surflets are basically scsh modules that are loaded
dynamically during run time.

\begin{alltt}
  (open surflets
        scheme-with-scsh)
\end{alltt}

The \name{open} form lists all the modules the \surflet needs.  You
will probably always need the two modules that are stated here
(namely \name{surflets} and \name{scheme-with-scsh}).  If you need
other modules, like \name{srfi-13} for string manipulation, this is
the place where you want to state it.

\begin{alltt}
  (begin
\end{alltt}

This just opens the body of the \surflet.  All your \surflet code goes
here.\footnote{If you know about scsh modules, you probably also know
that there is a \name{file} clause that you could use to place the
code in a file instead or along with the \name{begin} clause.}

\begin{alltt}
    (define (main req)
\end{alltt}

Here is the \name{main} function that the interface declared this
\surflet will implement.  The \name{main} function is the entry point
to your \surflet:  The server calls this function every time a user
browses to your \surflet the first time.  The server calls \name{main}
with one argument: a representation of the inital request of the
browser.  We don't have to worry about that at this point.

\begin{alltt}
      (send-html/finish
       '(html (body (h1 "Hello, world!")))))
    ))
\end{alltt}

\name{send-html/finish} is one of three function you will regularly
use to send web pages to the browser.  The other two functions are
\name{send-html} and \name{send-html/suspend}.
\name{send-html/finish} -- as the name already suggests -- sends a
HTML page to the browser and finishes the \surflet.  \name{send-html}
just sends the HTML page and does not return. 
\name{send-html/suspend} sends the HTML page and suspends the
\surflet, \ie it waits until the user continues with the \surflet, 
\eg by submitting a webform.  We will discuss \name{send-html} and
\name{send-html/suspend} in detail later.  I will refer to these three
functions as the \emph{sending functions}.

In a \surflet, HTML pages are represented as lists, or, to be more
precise, as SXML (S-expression based XML).\label{sec:SXML} The first
element of a SXML list is a symbol stating the HTML tag.  The other
elements of a SXML list are the contents that are enclosed by this
HTML tag.  The contents can be other SXML lists, too.  Here are some
examples of SXML lists and how they translate to HTML:

%\newcommand{\htmltag}[1]{$\mathtt{<}$#1$\mathtt{>}$}
\begin{tabular}{ll}
SXML: & \verb|'(p "A paragraph.")}|  \\
HTML: & \verb|<p>A paragraph.\htmltag{/p}}|\\
\\
SXML: & \verb|'(p "A paragraph." (br) "With break line.")}| \\
HTML: & \verb|<p>A paragraph.<br>With break line.</p>}|\\
\\
SXML: & \verb|'(p "Nested" (p "paragraphs"))}|\\
HTML: & \verb|<p>Nested<p>paragraphs</p></p>}|\\
\end{tabular}

Attributes are stated by a special list whose first element is the
at-symbol.  The attribute list must be the second element in the list:

\begin{tabular}{ll}
SXML: & \verb|'(a (@ (href "attr.html")) "Attributed HTML tags.")|\\
HTML: & \verb|<a href="attr.html">Attributed HTML tags.</a>|\\
\\
SXML: & \verb|'(a (@ (href "attr2.html") (target "\_blank")) "2
attributes.")}| \\ 
HTML: & \verb|<a href="attr2.html" target="\_blank">2 attributes.</a>}|
\end{tabular}

As you see from the \surflet example, \name{send-html/finish} expects
SXML as an argument.  In the example, the SXML translates to the
following HTML code:
\begin{alltt}
<html><body><h1>Hello, world!</h1>
</body>
</html>
\end{alltt}

Please note, that there is no check for valid HTML or even XHTML
here.  The only thing the translation process takes care of are
special characters in strings like the ampersand (\typew{\&}).  The
translation process replaces them by their HTML representation (\eg,
\typew{\&amp;}) so you don't have to worry about that when you use
strings.  Everything else like using valid HTML tags or valid
attributes is your responsibility.


\subsubsection{Dynamic content}

Let's extend our first \surflet example by some dynamic content, \eg
by displaying the current time using scsh's \name{format-date}
function.  As the HTML page is basically represented as a list, this
can be done like this:

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        scheme-with-scsh)
  (begin
    (define (main req)
      (send-html/finish
       `(html (body (h1 "Hello, world!")
                    (p "The current date and time is "
                       ,(format-date "~H:~M:~S ~p ~m/~d/~Y" 
                                     (date)))))))
    ))
\end{listing}

This \surflet can be found in \name{howto/hello-date.scm}.  The
beginning of this \surflet is the same as in the previous example.
The difference lies in the argument to \name{send-html/finish}.  Note
that the argument starts with a backquote (\typew{`}) rather than with
a regular quote (\typew{'}) as in the previous example.

Instead of passing a ``static'' list, \ie a list whose contents are
given before execution, this \surflet uses the quasiquote and unquote
feature of Scheme to create a ``dynamic'' list, \ie a list whose
contents are given only during execution.  A ``dynamic'' list is
introduced by a backquote (\typew{`}) and its dynamic contents are
noted by commata (\typew{,}).  Thus, if the \surflet is executed while
I am writing this howto, the argument to \name{send-html/finish} above
is translated to

\begin{alltt}
       '(html (body (h1 "Hello, world!")
                    (p "The current date and time is "
                       "13:09:03 PM 11/18/2003")))))
\end{alltt}

\noindent\emph{before} it is passed to \name{send-html/finish}.  Thus, using
dynamic content can be easily done with Scheme's quasiquote and
unquote feature.  Of course, you can build your list in any way you
want; the quasiquote notation is just a convenient way to do it.


\subsubsection{Several web pages in a row}

The previous example \surflets only showed one page and finished
afterwards.  Here, we want to present two web pages in a row.  We use
the previously mentioned function \name{send-html/suspend}, which
suspends after it has sent the page and continues when the user
clicked for the next page.  In contrast to \name{send-html/finish},
that expected SXML, \name{send-html/suspend} expects a function that
takes an argument and returns SXML.  The parameter the function gets
(here: \name{k-url}) is the URL that points to the next
page:\footnote{In the API this URL is called the \emph{continuation
URL}.}

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        scheme-with-scsh)
  (begin
    
    (define (main req)
      (send-html/suspend
       (lambda (k-url)
         `(html (body (h1 "Hello, world!")
                      (p (a (@ (href ,k-url)) "Next page -->"))))))
      (send-html/finish
       '(html (body (h1 "Hello, again!")))))
    ))
\end{listing}

This \surflet can be found in \name{howto/hello-twice.scm}.  This
example first displays a web page with the message ``Hello, world!''
and a link to the next page labeled with ``Next page --$>$''.  When the
user clicks on the provided link, \name{send-html/suspend} returns and
the next statement after the call to \name{send-html/suspend} is
executed.  Here it is \name{send-html/finish} which shows a web page
with the message ``Hello, again!''.

When \name{send-html/suspend} returns, (almost) the complete context
of the running \surflet is restored.  Thus, every variable in the
\surflet will retain its value during suspension.  The consequence is
that you don't have to worry about sessions, sesssion variables and
alike.  The user can freely use the back button of her browser or
clone a window while the \surflet will keep on responding in the
expected way.  This is all automatically managed by the
\surflet-handler.  

The only exception are variables whose values are changed by side
effects, \eg if you change a variable via \name{set!}.  These
variables keep their modified values, allowing communication between
sessions of the same \surflet.\footnote{If you want to change a
variable via side effects but you don't want to interfere with other
sessions, you can use \name{set-session-data!} and
\name{get-session-data}.  See the API documentation in section
\ref{sec:surflet-api} for further information.}


\subsubsection{Begin and end of sessions}

So far I don't have mentioned too much details about sessions.  The
reason is, as mentioned before, that the \surflet handler takes care
of the session automatically as described in the previous paragraph.
%, \ie it starts the session automatically when an
%instance of your \surflet starts and takes care of the saving and
%restoring of all variable values during suspensions of your \surflet
%instance, except for \typew{set!}ed values.

The only thing you have to worry about is when your session
\emph{ends}.  As long as your session hasn't been finished by
\name{send-html/finish}, the user can move freely between the web
pages your \surflet provides.  Once you've finished the session via
\name{send-html/finish}, this freedom ends.  As the session is over,
the user will get an error message when he tries to recall some web
page from the server.  The server will tell the user about the
possible reasons for the error (namely that most likely the session
was finished) and provides a link to the beginning of a new session.

Thus, \name{send-html/suspend} suspends the current execution of a
\surflet, returning with the request for the next web page of your
\surflet and \name{send/finish} finishes the session.  The third
sending function is \name{send-html} which just sends a web page.
\name{send-html} does not return and does not touch the session of
your \surflet instance.

\subsubsection{Abbreviations in SXML}
\label{sxml-abbrvs}

The example in subsection ``Several web pages in a row'' wrote down
the link to the next web page explicitly via the ``a''-tag.  As
websites contain a lot of links, the sending functions (like
\name{send-html/finish}) allow an abbreviation.  The following SXML
snippets are equivalent:

\begin{alltt}
(a (@ (href ,k-url)) "Next page -->")
(url ,k-url "Next page -->")
\end{alltt}

\name{url} expects the target address as the next element and includes
every text afterwards as part of the link. 

There are also some other abbreviations.  \typew{(nbsp)} inserts
`\typew{\&nbsp;}' into the HTML, \typew{(*COMMENT* \dots)} inserts a
comment, and with \typew{(plain-html \dots)} you can insert arbitrary
HTML code (\ie strings) directly , without any string conversions.
The last abbreviation, \name{surflet-form}, is discussed in the next
section.


\subsection{How to write web forms}

The \surflets come along with a libary for easy user interaction.  The
following subsections will show how to write web forms and how to get
the data the user has entered.


\subsubsection{Simple web forms}

Let's write a \surflet that reads user input and prints it out on the
next page:

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        scheme-with-scsh)
  (begin
    (define (main req)
      (let* ((text-input (make-text-field))
             (submit-button (make-submit-button))
             (req (send-html/suspend
                   (lambda (k-url)
                     `(html 
                       (body 
                        (h1 "Echo")
                        (surflet-form ,k-url
                                      (p "Please enter something:"
                                         ,text-input
                                         ,submit-button)))))))
             (bindings (get-bindings req))
             (user-input (input-field-value text-input bindings)))
        (send-html/finish
         `(html (body
                 (h1 "Echo result")
                 (p "You've entered: '" ,user-input "'."))))))
))
\end{listing}

Here are the details to the code in \name{main}:

\begin{alltt}
    (define (main req)
      (let* ((text-input (make-text-field))
             (submit-button (make-submit-button))
\end{alltt}

\name{make-text-field} and \name{make-submit-button} define two user
interaction elements: a text input field and a submit button.
\surflets represent user interaction elements by \name{Input-field}
objects.  Thus, user interaction elements are first class values in
\surflet, unlike in many other web scripting languages, \eg Java
surflets, PHP or Microsoft Active Server Pages, \ie you have a
representation of a user interaction element in your program that you
can pass to functions, receive them as return values, etc.  You'll
soon see the advantages of this approach.

\begin{alltt}
           (req (send-html/suspend
                 (lambda (k-url)
                   `(html 
                     (body 
                      (h1 "Echo")
                      (surflet-form ,k-url
                                    (p "Please enter something:"
                                       ,text-input
                                       ,submit-button)))))))
\end{alltt}

Instead of discarding the return value of \name{send-html/suspend} as
in the examples of the previous section, this time we'll save the
return value, as it will contain the data the user has entered in our
text input field.

The definition of the website is as described in the previous section
except for the new abbreviation \name{surflet-form}.
\name{surflet-form} creates the HTML code for a web form and expects
as its next value the URL to the next webpage as provided by
\name{send-html/suspend}, here named
\name{k-url}.  The remaining arguments constitute the content of the
web form.  Thus, the code above is equal to the following SXML:

\begin{alltt}
(form (@ (action ,k-url) (method "GET"))
  (p "Please enter something:"
     ,text-input
     ,submit-button))
\end{alltt}

If you want to use the POST method instead of the default GET method,
add the symbol \name{'POST} after the URL:

\begin{alltt}
(surflet-form ,k-url
              POST
              (p "Please enter something:"
                 ,text-input
                 ,submit-button))
\end{alltt}

The web page \name{send-html/suspend} sends to the browser looks like
in figure [missing]. 
%\ref{fig:user1-1}.  
After the user has entered his data into
the web form, \name{send-html/suspend} returns with the request object
of the browser for the next page.  This request object contains the
data the user has entered.

\begin{alltt}
             (bindings (get-bindings req))
\end{alltt}

With the function \name{get-bindings} we pull out the user data of the
request object.  Here we save the user data into the variable
\name{bindings}.  \name{get-bindings} works for both request methods
\typew{GET} and \typew{POST}.

\begin{alltt}
           (user-input (input-field-value text-input bindings)))
\end{alltt}

With the function \name{input-field-value} and the extracted user data
we can read the value for an \name{input-field}.   Here, we want to
know what the user has entered into the \name{text-input-field}.

\begin{alltt}
        (send-html/finish
         `(html (body
                 (h1 "Echo result")
                 (p "You've entered: '" ,user-input "'."))))))
\end{alltt}

After we have extracted what the user has entered into the text field,
we can show the final page of our \surflet and echo her input.

Thus, the scheme for user interaction is about the following:

\begin{itemize}
\item Create the user interaction elements, \name{input-field}s, you
want to use in your web page.
\item Send the web page with \name{send-html/suspend} to the browser.
Plug in the \name{input-field}s in the web page as if they were usual
values.  Save the return value of \name{send-html/suspend}.
\item Extract the user data from the return value of
\name{send-html/suspend}.
\item Read the values of each \name{input-field} out of the extracted
user data with \name{input-field-value}.
\end{itemize}

The complete list of functions that create \name{input-fields} can be
found in the API in section \ref{sec:surflet-api}.

\subsubsection{Return types other than strings}
\label{subsec:input-return}

As the user interaction elements are first class values in a \surflet,
they can return other types than strings.  For example the \surflets
come with a number input field, \ie an input field that accepts only
text that can be interpreted as a number.  If the user enters
something that is not a number, \name{input-field-value} will return
\sharpf as the value of the number input field.  If you'd rather want
an error to be raised, you can use \name{raw-input-field-value}
instead.

\subsubsection{Annotated input fields}

The return value of an input field need not even be a primitive
value.  The \surflets library allows you to ``annotate'' your input
fields with values which should be returned indicated by the user's
input.  \Eg, consider this \surflet:

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
	handle-fatal-error
        scheme-with-scsh)
  (begin
    (define (main req)
      (let* ((select-input-field 
              (make-select
               (map make-annotated-select-option
                    '("Icecream" "Chocolate" "Candy")
                    '(1.5 2.0 0.5))))
              (req (send-html/suspend
                    (lambda (k-url)
                      `(html 
                        (head (title "Sweet Store"))
                        (body
                         (h1 "Your choice")
                         (surflet-form 
                          ,k-url
                          (p "Select the sweet you want:"
                             ,select-input-field)
                          ,(make-submit-button)))))))
              (bindings (get-bindings req))
              (price (input-field-value select-input-field 
                                        bindings)))
        (send-html/finish
         `(html (head (title "Receipt"))
                (body
                 (h2 "Your receipt:")
                 (p "This costs you \$" ,price "."))))))
))
\end{listing}

Let's go through the important part of this \surflet:

\begin{alltt}
      (let* ((select-input-field 
              (make-select
               (map make-annotated-select-option
                    '("Icecream" "Chocolate" "Candy")
                    '(1.5 2.0 0.5))))
\end{alltt}

Here we define a select input field (a dropdown list).  Instead of
only providing a list of values that shall show up in the dropdown
list and later examining which one was selected and looking up the
price for the sweet, we bind the values in the list with the price
while we create the select input field.  When the select input field
is shown in the browser, it will show the names of the sweets.  When
we lookup the user's input, we will get the associated price for the
sweet.  Again, this works not only with numbers, but with any
arbitrary Scheme value (\eg functions or records).

\subsubsection{Sending error messages}

If a user tries to forge a \surflet-URL (\eg by extracting the
continuation URL from the HTML source and editing it), your \surflet
has to deal with unexpected values.  Usually, a forged \surflet-URL
will result in an error that is raised in one of the \surflet library
functions.  If you don't catch this error, the \surflet handler will
catch it for you, send an error message to the user
\emph{and terminate the current session} as your \surflet obviously
encountered an unexpected error and might be in an invalid state.  If
you don't want this behavior, you can catch this error (like any other
error that is raised by \scsh) and send your own error message with
\name{send-error} which is located in the \name{surflets/error}
package.  The \name{handle-fatal-error} package can be useful in this
context.  Here's an example, that modifies the example from the
previous subsection (modifications emphasized):

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
\codemph{        handle-fatal-error}
\codemph{        surflets/error}
        scheme-with-scsh)
  (begin
    (define (main req)
      (let* ((select-input-field 
              (make-select
               (map make-annotated-select-option
                    '("Icecream" "Chocolate" "Candy")
                    '(1.5 2.0 0.5))))
              (req (send-html/suspend
                    (lambda (k-url)
                      `(html 
                        (head (title "Sweet Store"))
                        (body
                         (h1 "Your choice")
                         (surflet-form 
                          ,k-url
                          (p "Select the sweet you want:"
                             ,select-input-field)
                          ,(make-submit-button)))))))
              (bindings (get-bindings req))
\codemph{              (cost (with-fatal-error-handler               }
\codemph{                     (lambda (condition decline)            }
\codemph{                       (send-error (status-code bad-request)}
\codemph{                                   req                      }
\codemph{                                   "No such option or internal }
\codemph{                                    error. Please try again."))}
\codemph{                     (raw-input-field-value select-input-field }
\codemph{                                            bindings))))    }
        (send-html/finish
         `(html (head (title "Receipt"))
                (body
                 (h2 "Your receipt:")
                 (p "This costs you \$" ,cost "."))))))
))
\end{listing}

Let's examine the important part of this example:

\begin{alltt}
              (cost (with-fatal-error-handler
                     (lambda (condition decline)
                       (send-error (status-code bad-request)
                                   req
                                   "No such option or internal
                                    error. Please try again."))
                     (raw-input-field-value select-input-field 
                                            bindings))))
\end{alltt}

As mentioned in \ref{subsec:input-return}, this \surflet uses
\name{raw-input-field-value} instead of \name{input-field-value}
because the former raises an error while the latter returns \sharpf\ in
case of an error.

If a user forges a continuation URL, \name{raw-input-field-value}
might not be able to find a valid value for the
\name{select-input-field} and raises an error.  This error is catched
by the error handler which was installed by
\name{with-fatal-error-handler}.  The error handler uses
\name{send-error} to send an error message to the browser.  Its first
argument is the status code of the error message.  See the
documentation of the \sunet webserver for different status codes.  The
second argument is the request which was processed while the error
occured.  The last argument is a free text message to explain the
cause of the error to the user.

While in the original \surflet the user will still see the resulting
receipt web page with an empty dollar amount and has her session
finished, this modified version will show an error message and won't
finish the session.

It is your choice, which version you choose, \ie if you let the
\surflet handler handle the occuring error automatically or if you
install your own error handlers and use \name{raw-input-field-value}.
However, be careful if you use \name{raw-input-field-value} along with
check boxes.  The HTML standard dictates that an unchecked check box
does not appear in the data the browser sends to the server.  Thus,
\name{raw-input-field-value} won't find the check box in the data and
raise an error which is not a ``real'' error as you migh expect it.


\subsubsection{Your own input fields}

The \surflet library contains constructors for all input fields that
are described in the HTML~2.0 standard.  See the \surflet API in
section \ref{sec:surflet-api} for a complete list.  The \surflet
library also allows you to create your own input fields, \eg an input
field that only accepts valid dates as its input.  This subsection
gives you a short overview how to do this.  You will find the details
in the \surflet API.

Let's have a look at an \surflet that uses its own input field.  The
``input field'', called nibble input field, consists of four check
boxes which represent bits of a nibble (half a byte).  The value of
the input field is the number that the check boxes represent.  \Eg, if
the user checks the last two checkboxes, the value of the nibble input
field is 3.

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        surflets/my-input-fields
        scheme-with-scsh)
  (begin

    (define (make-nibble-input-fields)
      (let ((checkboxes (list (make-annotated-checkbox 8)
                              (make-annotated-checkbox 4)
                              (make-annotated-checkbox 2)
                              (make-annotated-checkbox 1))))
        (make-multi-input-field
         #f "nibble-input"
         (lambda (input-field bindings)
           (let loop ((sum 0)
                      (checkboxes checkboxes))
             (if (null? checkboxes)
                 sum
                 (loop (+ sum (or (input-field-value (car checkboxes) 
                                                     bindings)
                                  0))
                       (cdr checkboxes)))))
         '()
         (lambda (ignore)
           checkboxes))))

    (define nibble-input-field (make-nibble-input-fields))

    (define (main req)
      (let* ((req (send-html/suspend
                   (lambda (new-url)
                     `(html (title "Nibble Input Widget")
                            (body 
                             (h1 "Nibble Input Widget")
                             (p "Enter your nibble (msb left):")
                             (surflet-form ,new-url
                                           ,nibble-input-field
                                           ,(make-submit-button)))))))
             (bindings (get-bindings req))
             (number (input-field-value nibble-input-field bindings)))
        (send-html
         `(html (title "Result")
                (body 
                 (h2 "Result")
                 (p "You've entered " ,number "."))))))
    ))
\end{listing}

Let's go through this \surflet step by step.

\begin{alltt}
(define-structure surflet surflet-interface
  (open surflets
        surflets/my-input-fields
        scheme-with-scsh)
\end{alltt}

If you want to create your own input fields, you have to open the
\name{surflets/my-input-fields} package.

\begin{alltt}
  (begin
    (define (make-nibble-input-fields)
      (let ((checkboxes (list (make-annotated-checkbox 8)
                              (make-annotated-checkbox 4)
                              (make-annotated-checkbox 2)
                              (make-annotated-checkbox 1))))
\end{alltt}

\name{make-nibble-input-fields} is the constructor for our new type of
input field.  As mentioned before, we use check boxes to let the user
enter the nibble.  We use annotated checkboxes for this purpose whose
value is the value in the nibble.

\begin{alltt}
        (make-multi-input-field
         #f "nibble-input"
\end{alltt}

The value of our new input field will depend on the value of several
real input fields.  Thus we create a multi input field.  If the value
depended only on the browser data that is associated to the name of
our input field, we would use \name{make-input-field} instead, which
creates a usual input field.  \Eg, if we wanted to create a date input
field that accepts only valid dates as input and used a text input
field for this purpose, we would use \name{make-input-field}.

The first two parameters is the name of the input field and its type.
As we use checkboxes to represent our input field, we don't need the
name field.  The type field is meant for debugging purposes, so you
can identify the type of the input field during a debugging session.

\begin{alltt}
         (lambda (input-field bindings)
           (let loop ((sum 0)
                      (checkboxes checkboxes))
             (if (null? checkboxes)
                 sum
                 (loop (+ sum (or (input-field-value (car checkboxes) 
                                                     bindings)
                                  0))
                       (cdr checkboxes)))))
\end{alltt}

The next parameter is the so called transformer function.
\name{raw-input-field-value} calls the transformer function to
determine the value of the input field depending on the given
bindings.  The transformer function of a multi input field (which our
nibble input field is) gets the input field and the bindings as
parameters.  A usual input field would only get the data that is
associated to its name.

The transformer function of our nibble input field goes over each
check box, looks it up in the bindings and adds its value to a sum, if
\name{input-field-value} can find it.  If it can't find it, zero is
added instead.  The value of our nibble input field is the resulting
sum.

The rest of the \surflet is straight forward and not repeated here
again.  We create, use and evaluate the nibble input field as we do
with every other input field.


\subsection{Program flow control}

With the techniques shown so far it is rather difficult to create a
web page that has several different successor webpages rather than
only one web page.  This section will show you how to do this with the
\surflets.  Basically, there are two different methods how to perform
this task.  One method is to mark each link in some way and evaluate
the mark after \name{send-html/suspend} has returned.  The other
method is to bind a callback function to each link that is called when
the user selects the link.  This section shows both methods.

\subsubsection{Dispatching to more than one successor web page}

The basic idea of dispatching is to add a mark to a link and evaluate
it after the user has clicked on a link and \name{send-html/suspend}
returned.  Let's have a look at an example.  It shows an entry page at
which the user states the language in which she wants to be greeted:

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        scheme-with-scsh)
  (begin

    (define (main req)
      (let* ((english (make-address))
             (german (make-address))
             (req (send-html/suspend
                   (lambda (k-url)
                     `(html 
                       (head (title "Multi-lingual"))
                       (body 
                        (h2 "Select your language:")
                        (ul
                         (li (url ,(english k-url) "English")
                         (li (url ,(german k-url) "Deutsch")))))))))
             (bindings (get-bindings req)))
        (case-returned-via bindings
          ((english) (result-page "Hello, how are you?"))
          ((german) (result-page "Hallo, wie geht es Ihnen?")))))

    (define (result-page text)
      (send-html/finish
       `(html 
         (head (title "Greeting"))
         (body
          (h2 ,text)))))
    ))
\end{listing}

Let's see how \name{main} presents the different options:

\begin{alltt}
    (define (main req)
      (let* ((english (make-address))
             (german (make-address))
\end{alltt}

Of course you don't have to worry about adding the mark to the links.
Instead, we create the links with \name{make-address}.

\begin{alltt}
             (req (send-html/suspend
                   (lambda (k-url)
                     `(html 
                       (head (title "Multi-lingual"))
                       (body 
                        (h2 "Select your language:")
                        (ul
                         (li (url ,(english k-url) "English")
                         (li (url ,(german k-url) "Deutsch")))))))))
\end{alltt}

\name{make-address} returns a function you can call to create the
link as we did here with

\begin{alltt}
 (li (url ,(english k-url) "English")
\end{alltt}

This creates a list item which contains a hyperlink labeled
``English''.  The hyperlink is created by the SXML abbreviation
\name{url} as shown in \ref{sxml-abbrvs}.  Instead of just passing 
the continuation URL \name{k-url} to \name{url}, we create the marked
link by calling the function \name{make-adddress} gave us.

\begin{alltt}
             (bindings (get-bindings req)))
        (case-returned-via bindings
          ((english) (result-page "Hello, how are you?"))
          ((german) (result-page "Hallo, wie geht es Ihnen?")))))
\end{alltt}

After \name{send-html/suspend} has returned, we can evaluate which
link the user has clicked by using \name{case-returned-via}.
\name{case-returned-via} works similar to the regular \name{case} of
Scheme.  It evaluates the body of the form whose initial list contains
the address that the user used to leave the website.  \Eg, if the user
has selected ``German'' as her preferred language and clicked on the
link we have named \name{german} in our \surflet,
\name{case-returned-via} will evaluate its second form and the
\surflet will display the greeting in German.

\name{case-returned-via} is syntactic sugar like the regular
\name{case}.  However, instead of \name{equal?} it uses
\name{returned-via}.  \name{returned-via} takes the bindings and 
and an address and returns \sharpt, if the user left the web page via
this address (\ie, via the link that is represented by this address)
and \sharpf otherwise.  \name{returned-via} does not end with a
question mark as it might return other values as well as we will see
shortly.  Of course, it is your choice if you want to use
\name{case-returned-via} or explicitly \name{returned-via}.


\subsubsection{Annotated dispatching}

The approach shown in the previous subsection has one major drawback:
the meaning of an address becomes clear only when you look at the
dispatching section of \name{case-returned-via}.  This subsection
shows you how to link the meaning and the representation of an address
closer together.

We modify the previous code example slightly to this \surflet
(differences emphasized):

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        scheme-with-scsh)
  (begin
    
    (define (main req)
      (let* (\codemph{(language (make-annotated-address))}
             (req (send-html/suspend
                   (lambda (k-url)
                     `(html 
                       (head (title "Multi-lingual"))
                       (body 
                        (h2 "Select your language:")
                        (ul
                         (li (url ,\codemph{(language k-url }
\codemph{                                             "Hello, how are you?")} 
                                  "English")
                         (li (url ,\codemph{(language k-url }
\codemph{                                             "Hallo, wie geht es Ihnen?")}
                                  "Deutsch")))))))))
             (bindings (get-bindings req)))
        (case-returned-via bindings
\codemph{          ((language) => result-page))))}

    (define (result-page text)
      (send-html/finish
       `(html 
         (head (title "Greeting"))
         (body
          (h2 ,text)))))
    ))
\end{listing}

Let's look at the differing parts:

\begin{alltt}
      (let* ((language (make-annotated-address))
\end{alltt}

To link the meaning with the address itself, we use an annotated
address.  As we can annotate the address now, we don't need two
distinct addresses anymore.

\begin{alltt}
          (li (url ,(language k-url 
                              "Hello, how are you?")
                   "English")
          (li (url ,(language k-url 
                              "Hallo, wie geht es Ihnen?")
                   "Deutsch")))))))))
\end{alltt}

In addition to the continuation URL \name{k-url} we also annotate the
address with a value.  Here we use the different greetings as the
annotation.  The address can be annotated with any arbitrary Scheme
value, \eg functions or records.

\begin{alltt}
        (case-returned-via bindings
          ((language) => result-page))))
\end{alltt}

\name{case-returned-via} has an extended syntax similar to \name{cond}
that it useful with annotated address.  The arrow `\name{=>}' calls
the following function with the annotation of the address via which
the user has left the web page.  You can extract the annotation
yourself with \name{returned-via} like this:

\begin{alltt}
(result-page (returned-via language bindings))	
\end{alltt}

This will call \name{result-page} with the annotation of the address
via which the user has left the web page.  \name{returned-via} returns
\sharpf, if the user didn't leave the web page via one of the links
created with this address (which is not really possible in this
example).

\subsubsection{Callbacks}

The other method to lead to different successor web pages is using
callbacks.  A callback is a function that is called if the user leaves
the web page via an associated link.  This is different from the
dispatch method where \name{send-html/suspend} returns.  You can
create a web page that only uses callbacks to lead to successor web
pages and you don't have to use \name{send-html/suspend}.
Instead, you can use \name{send-html}.

Although it is possible to use several different callbacks in a single
web page, this is not recommended.  The reason lies in the
implementation of a callback, which saves the current continuation.
Several different callbacks would result in the storage of the several
slightly different continuations.  This is unnecessary, as you can
annotate the callbacks with the arguments for the callback function.
Let's see an example which is a variation of the previous examples
(important parts / differences emphasized):

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        \codemph{surflets/callbacks}
        scheme-with-scsh)
  (begin
    
    (define (main req)
      (let ((language \codemph{(make-annotated-callback result-page)}))
        (\codemph{send-html}
         `(html 
           (head (title "Multi-lingual"))
           (body 
            (h2 "Select your language:")
            (ul
             (li (url ,\codemph{(language "Hello, how are you?")}
                      "English")
                 (li (url ,\codemph{(language "Hallo, wie geht es Ihnen?")}
                          "Deutsch")))))))))

    (define \codemph{(result-page req text)}
      (send-html/finish
       `(html 
         (head (title "Greeting"))
         (body
          (h2 ,text)))))
    ))
\end{listing}

The differences to the dispatch method are the following: you have to
open the \name{surflets/callbacks} package to use callbacks, you don't
use the continuation URL to create the callback link, and the
callbacked function must accept the request from the browser as the
first argument.  Furthermore, you don't have to use
\name{send-html/suspend}, if a user can only leave your web page via
callbacks.  However, it can be sensible to combine the dispatch and
the callback method, in which case you have to use
\name{send-html/suspend}.

Note that is nonsensical to create a callback on top level, \ie the
call to \name{make-annotated-callback} must occur every time
\name{main} is called and not only once when the \surflet is read into
memory.  For the same reason it is nonsensical in most cases to reuse
a callback.

The \surflet library provides also a wrapper function with which you
can instruct the callback to call different functions instead of a
single one.  If you create your callback like

\begin{alltt}
(let ((callback (make-annotated-callback callback-function)))
   \dots)
\end{alltt}

you can instruct the callback to call different functions like this:

\begin{alltt}
(callback function1 arg1 arg2)
\dots \\
(callback function2 arg3 arg4 arg5)
\end{alltt}

Again, it is your choice which option you want to use.  Note that
calling a function with several arguments and of different amount each
time is also possible if you only use a single function for the
callback.

\subsection{Data management}

When you write web programs, there are usually two kinds of data that
you use: data that is local to each instance of a \surflet, \eg the
user's login, and data that is global to each instance of a \surflet,
\eg a port to a logfile.  Changes to local data is only visible to
each session of a \surflet, while changes to global data is visible
to every session of a \surflet.

The \surflet library does not really distinguish between these two
types of data, but provides ways to realize both of them in a
convenient way that is not (really) different from the way you handle
these data types in a regular Scheme program.

If a data item is globally used in your \surflet, define it global
(on top level) and change its values with \name{set!}.   If a data
item is locally used, define it locally (in your function) and do not
change its value with \name{set!}.  

If the following sounds too technical to you, you can safely skip this
paragraph.  The reason why the distinction between global and local
data is done via whether you mutate the data's value with \name{set!}
is that the \surflets are implemented with continuations.
Continuations cannot reflect changes that are done via \name{set!} (or
side effects in general) and thus such changes are globally visible.
On the other hand continuations represent states of a program and a
reified continuations reifies also the values of all data.

But what to do if you happen to want to change your \emph{local}
data's value with \name{set!}?  The \surflet library provides a place
where you store such mutable local data and two functions to access
it: \name{set-session-data!} sets the mutable local data and
\name{get-session-data} reads the mutable local data.  Here is an
example.  It uses the callback technique that was presented in the
previous section.  If you haven't read that section, you only need to
know that \name{show-page} is called again and again as long as the
user keeps on clicking on ``Click''.

\begin{listing}
(define-structure surflet surflet-interface
  (open surflets
        surflets/callbacks
        scheme-with-scsh)
  (begin
    (define (main req)
      (set-session-data! -1)
      (let ((start (make-annotated-callback show-page)))
        (show-page req 'click start)))

    (define (show-page req what callback)
      (if (eq? what 'click)
          (click callback)
          (cancel)))

    (define (click callback)
      (set-session-data! (+ 1 (get-session-data)))
      (send-html
       `(html 
         (head (title "Click counter"))
         (body
          (h2 "Click or cancel")
          (p "You've already clicked " 
             ,(get-session-data) 
             " times.")
          (p (url ,(callback 'click callback) "Click")
             (url ,(callback 'cancel callback) "Cancel"))))))

    (define (cancel)
      (send-html/finish
       `(html
         (head (title "Click counter finished"))
         (body
          (h2 "Finished")
          (p "after " ,(get-session-data) " clicks.")))))
))
\end{listing}

At the beginning of \name{main} we initialize the mutable local data
with \name{set-session-data!}.

\begin{alltt}
    (define (main req)
      (set-session-data! -1)
      (let ((start (make-annotated-callback show-page)))
        (show-page req 'click start)))
\end{alltt}

Afterwards, we create and save a callback that will be called again
and again.  We call \name{show-page} with the callback to show the
first web page.

\begin{alltt}
    (define (show-page req what callback)
      (if (eq? what 'click)
          (click callback)
          (cancel)))
\end{alltt}

\name{show-page} evaluates its second argument \name{what} to
determine what to do next: continue or cancel.

\begin{alltt}
    (define (click callback)
      (set-session-data! (+ 1 (get-session-data)))
      (send-html
       `(html 
         (head (title "Click counter"))
         (body
          (h2 "Click or cancel")
          (p "You've already clicked " 
             ,(get-session-data) 
             " times.")
          (p (url ,(callback 'click callback) "Click")
             (url ,(callback 'cancel callback) "Cancel"))))))
\end{alltt}

If the user continues, \name{click} increases the mutable local
counter and shows the next page.  

Note that we don't use \name{send-html/suspend} here because we use
the callback to lead to the next web page.  If the user clicks on the
link labeled with ``Click'' or ``Cancel'', \name{show-page} will be
called with \typew{'click} or \typew{'cancel}, respectively, and the
callback as parameters.  This creates an endless loop without saving
endless states of the \surflet.

\name{cancel} shows the final page with the amount of clicks
performed.  

\subsection{My own SXML}

Section \ref{sec:SXML} introduced SXML, the way how \surflets
represent HTML.   This section will show you, how you can create your
own rules to translate from SXML to HTML.

\subsubsection{Terms and theoretical background}

This subsection will introduce the main concepts of the translation
process and some necessary terms we will use in the following.

The translation process from SXML to HTML takes two steps.  In the
first step, SXML is translated to an intermediate form.  This is done
by the \textit{translator}.  In the second step, the intermediate form
is printed into an HTML string.  This is done by the
\textit{printer}.  The intermediate form looks very much like SXML,
but contains only \textit{atoms} or, recursively, list of atoms.
Atoms are numbers, characters, strings, \sharpf, and the empty list.
We call the intermediate form an \textit{atom tree} and the list from
which we've started an \textit{SXML tree}.

The basic unit in the translation process is a \textit{conversion
rule}.  A conversion rule consists of a trigger and a conversion
function.  As its first element, the trigger identifies the list for
which the translator shall call the conversion function.  The
translator calls the conversion function with all list elements as
parameters and replaces the whole list by the result of the conversion
function.  The result of the conversion function is supposed to be an
atom tree.

The translator takes the SXML tree and a list of conversion rules as
arguments.  It then traverses the SXML tree depth first and calls the
conversion functions according to the triggers it encounters,
replacing the nodes in the SXML tree with the return values of each
conversion function called.  The result of this translation step will
be an atom tree, which the printer will print into a string or port.

The translator calls the conversion function in two different modes,
depending on the conversion rule.  The regular mode is the
\textit{preprocess} mode:  the translator translates every argument of
the conversion function before calling it.  The other mode is the
\textit{unprocessed} mode:  the translator calls the conversion
function directly without preprocessing the arguments.  This is, the
translator stops traversing the SXML tree at nodes that trigger a
conversion rule in unprocessed mode.  

There are two default triggers which you can't use in your translation
rules: \typew{*default*} and \typew{*text*}.  \typew{*default*} as the
trigger marks the default conversion rule which the translator uses if
no other conversion rule triggers.  \typew{*text*} marks the text
conversion rule and triggers, if the node in the SXML tree is a
string.  In the standard conversion rule set the text conversion rule
performs HTML escaping, \eg for the ampersand (\&).


\subsubsection{Outlook}

More to come soon about \surflets consisting of different parts and
individual SXML.



%\end{document}

%session data
%surflets containing of different parts
%my sxml