Processing forms using scheme
Required Reading
Servlets
A servlet is a program that runs on a web server and which
dynamically generates webpages. It often will use data submitted
by the user on HTML forms to generate the new webpage, and it
may also access a database and/or send email while storing the data.
All of the servlets we will examine in this class have a URL
of the following form:
<![CDATA[
http://tat.cs.brandeis.edu:8080/user/Cosi2a/USERNAME/FILENAME.sxml
]]>
where the extension "sxml" tells the server that this is a servlet.
Forms and parameters
Here are some examples of simple servlets, together with the
html code for invoking them:
- bounce
This servlet just shows you the values of the parameters you have sent.
It is useful when designing forms.
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<![CDATA[
<form
method=post
action=
"http://tat.cs.brandeis.edu:8080/user/classes/aut01/cs2a/tjhickey/bounce.sxml">
<ul>
<li> Enter your country of birth:
<input type=text name=name value="USA">
<li> Enter your age:
<input type=text name=age value="18">
<li> Enter your passphrase:
<input type=password name=pw value="18">
<li> Write your bio:
<textarea name=bio rows=5>
I was born in ...
at ..
</textarea>
</ul>
<input type=submit name=submitbutton value=send>
<input type=reset value=clear>
<input type=submit name=dosomething value=hmmm>
</form>
]]>
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-
the date servlet
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Clicking on
this hyperlink
causes the servlet to generate
a webpage containing the current time!
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<![CDATA[
Clicking on
<a href="http://tat.cs.brandeis.edu:8080/user/classes/aut01/cs2a/tjhickey/date.sxml">
this hyperlink
</a> causes the servlet to generate
a webpage containing the current time!
]]>
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The code for the date servlet is
<![CDATA[
<html>
<head><title>Date</title></head>
<body bgcolor=black text=red>
<center>
<font size=+5>
It is now <scheme>(Date.)</scheme>
</font>
</center>
</body>
</html>
]]>
Notice the use of the <scheme> tag
to include some scheme code in the HTML.
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SXML: a language for writing servlets
In this class, we will use the SXML dialect of Scheme to
create servlets. The simplest form of such a program will be
the following
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<![CDATA[
(let ((v1 (param request "name1"))
(v2 (param request "name2"))
...
(vn (param request "namen"))
)
<xml>
<html
<head> <title> ...</title></head>
<body>
...regular html code
<scheme>v3</scheme>
....
<scheme>v2</scheme>
...
</body>
</html>
</xml>
)
]]>
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In this program the form data is first read and stored in
the variables v1, v2, ..., vn. Then then HTML below
is generated and the form data is inserted into the HTML
by the vi expressions.
In general, you can have any scheme expression inside the scheme
tags. A common idiom is to have some scheme code use a test to decide
which bit of xml to insert. For example the following program generates
two different pages depending on whether the correct password was entered.
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<![CDATA[
(let ((pw (param request "passwd"))
)
<xml>
<html
<head> <title>Password test</title></head>
<body>
<h1>Password test</h1>
<scheme>
(if (equal? pw 'john34mary)
<xml>You know the password, please go
<a href="http://silk.cs.brandeis.edu">here</a>
for more info</xml>
<xml>Sorry, go ask the teacher for the password!</xml>
)
</scheme>
</body>
</html>
</xml>
)
]]>
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It could be called from a page of the form:
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<![CDATA[
<html
<head> <title>Password test</title></head>
<body>
<h1>Password test</h1>
<form method=post action=pw.sxml>
Enter the password <input type=password name=passwd size=20>
<p>
<input type=submit>
</form>
</body>
</html>
]]>
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By default if the form data consists of several words, (param expr "zzz"),
will return only the first "word". If you want the entire text as a
quoted string, you can use
(.getParameter request "zzz")
Similarly, some form elements allow you to select multiple data items.
By default (param request "zzz") only returns the first. If you want all
of them you can use:
(.getParametervalues request "zzz")
Example from class
In class we developed a single fairly complex example:
form1.html.
The HTML for the page gets the user's name, age, favorite color,
and password and sends it to the "demoB.sxml" page for processing.
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<![CDATA[
<html><body bgcolor=lightgreen>
<form method=post
action="demoB.sxml">
<ul>
<li>
Name: <input type=text name="who">
</li>
<li> Age: <input type=text name="age">
</li>
<li> Favorite color:
<select name="favcol">
<option> red </option>
<option> lightgreen </option>
<option> blue </option>
<option> yellow </option>
</select>
<li> Password:
<input type=password name="pw">
</li>
</ul>
<input type=submit>
</form>
</body>
</html>
]]>
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This servlet page
demoB.sxml.
then
- reads the four input parameters
(and converts the "age" parameter to a decimal number).
- checks to see if the password is "seven!", and
if the password is wrong, it generates an error page
- if the password is "seven!", it
- personalizes the page with their name
- generates a page using their favorite color as background
- computes their age in millions of seconds
as follows:
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<![CDATA[
(let (
(thecolor (.getParameter request "favcol"))
(person (.getParameter request "who"))
(age (Double.
(.getParameter request "age")))
(pw (.getParameter request "pw"))
)
(if
(not(equal? pw "seven!"))
<xml>
<html><body bgcolor=black text=red>
<h1>WRONG WRONG WRONG</h1></body></html>
</xml>
<xml>
<html>
<body bgcolor=
<scheme>thecolor</scheme>
>
Hello, your name is
<font color=red>
<scheme>person</scheme></font>
and your age is<font color=blue>
<scheme>age</scheme>
</font>
<br>
Your age is
<scheme>
(/ (* age 365 24 60 60) 1000000.0)
</scheme> million seconds.
See you.
</body>
</html>
</xml>
)
)
]]>
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