CGI programming
You want to write scripts that follow the Common Gateway Interface,
aka
CGI, for instance to request information to register new users to
your site.
There are two major components in writing scripts for the Web, the
first one is to be able to write the HTML that will
initiate the
execution of the script, and the other is to write the program that
will process the information received through the HTTP protocol.
HTTP forms
We will presume that you know the basics, so here's a very simple example:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0
Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head><title>Registration</title></head>
<body>
<h1>Registration</h1>
<p>To register as a new user, simply fill out this form:</p>
<form action="/servlets/doregister.ss" method="post">
<table border="0">
<tr><td align="right"> First & last name: </td>
<td><input type="text" name="Name" size="40" value=""/></td></tr>
<tr><td align="right"> Email address: </td>
<td><input type="text" name="Email" size="40" value="" /></td></tr>
<tr><td colspan="2" align="center"> <input type="submit" value=" Submit " /></td></tr>
</table>
</form>
</body>
</html>
This HTML form has two important bits of information, on the one hand
the
method used to send the information from the browser to the
server, in this case
post, which
packs the information and sends
it to the server once you hit the
Submit button.
On the other hand, we have the
action, in this case an scheme
program that will handle the request, process the registration, and
return to the client.
CGI programming in Scheme
PLT Scheme ships with a
CGI library,
cgi.ss, that will help you
handle this. Lets review some of the nomenclature, or types, used by
this library:
- binding
- an association of a form item with its value.
- bindings
- a list of binding's.
- html-string
- any text string that has been escaped according to HTML conventions.
It should be clear now that what we want to do in order to retrieve
the information provided by the user in the form is to
get the bindings, search for the values of the
Name, and
Email keys.
(let* ((b (get-bindings))
(name (extract-binding/single "Name" b))
(email (extract-binding/single "Email" b)))
We used
get-bindings, which returns the whole set of bindings
delivered by the client. Note that there is also a
get-bindings/post, and a
get-bindings/get, which only work when
the
method POST, and GET are used respectively, while
get-bindings
determines the kind of method used, and invokes the appropriate
procedure.
extract-binding/single returns the value associated to the key
passed as its first argument in the set of bindings passed as its
second argument. The result is either a string representing the
value, or a web page with an error, for instance when there was more
than one value associated to the key, or the key wasn't found.
When there is more than one value, or an unknown number of values,
associated to a single key, you should use
extract-bindings, which
returns a list with zero, one, or many associations for the key.
After getting, and processing the information (validate it, store it
in a database, etc.), you will want to return to the client. The
cgi.ss library provides one convenient way to do just that:
(generate-html-output
"Thank you!"
(list (format "Dear ~a, " name)
"We have received your information, and this completes the registration process. "
"Thank you!"))
And that's it. There are a couple of other procedures provided by the
cgi.ss library, but this should cover the basis.
Important: In order to actually be able to
run this example
(execute the small scheme script above), you need to have a Web
server properly configured, the script has to reside in the right
place and have the right permissions, and the URI depicted in the
action field,
"/servlets/doregister.ss", must point to it.
--
FranciscoSolsona - 06 Apr 2004
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