CS 4536 - Web Programming Assignment

CS 4536 Homework 5: Web Programming Option

Due: Thursday, Feburary 14, 11:59pm via turnin (assignment name hwk5)
Collaboration Policy: Pairs Permitted

The Assignment

For this assignment, you will implement the same blog editing program that we covered in class using the Scheme web API.

Expected program behavior: The sequence of screens is as in the class handout (with the exception that you should implement the choice of whether to accept using a radio button or check box, rather than a submit button). Specifically, the main page (editing) provides input areas for entering the title and contents of a new blog post, a "preview" button and a list of existing posts. Pressing the preview button brings up a page showing the new post, with date. This page has a radio or checkbox input indicating whether to accept the post and a "Continue" button. Pressing the continue button returns to the entry page. If the previewed post was accepted, the post now appears in the list of posts and the title and content areas are empty. If the post was not accepted, the text and contents areas contain the previously previewed text (for further editing). Your solution should satisfy all of the requirements listed on the class handout.

Here is the sketchy text version that I presented in class (missing details), as well as an annotated version of the adder as a Scheme web script.

Documentation on Writing Servlets in the Scheme Server

You can figure out most of this assignment using the add example linked above (same as the one passed out in class). The servlets directory (described below) has examples of other servlets that use more of the html features. You may find it handy to look at those. For documentation on the API, search for send/suspend in the DrScheme helpdesk. Use only the HelpDesk link under "Web Server Manual" -- the one under "Teachpacks" won't work.

Use the (module ...) language level if you are working in DrScheme. The usual PLAI language levels will insert metadata at the top of your file that the web server will subsequently choke on. The module language tells DrScheme to skip the metadata.

To use empty, first, etc, you'll need to change the require clause at the top of the add example to the following:

   (require (lib "servlet.ss" "web-server")
            (lib "list.ss"))

Learning HTML: You should be able to figure out the HTML needed for this assignment via examples. If you look at the pre-course survey and "View Source" in your web browser, for example, you can see how to create forms, input buttons, radio buttons, etc. You shouldn't need to spend lots of time reading HTML tutorials for this. Feel free to post questions to the board. Don't be embarrassed -- lots of students in the class are new to web programming.

Quasiquoting: you may notice sample servlets quoting lists with "`" instead of "'", called quasiquoting, as opposed to quoting. Quasiquoting provides the ability to force evaluation on specific elements of a quasiquoted expression instead of directly returning the expression. To force evaluation of an element in a quasiquoted expression, prefix the element with the character ",". For example, to generate the list '(1 2 3 4) you may write `(1 2 ,(+ 0 3) 4). Quasiquoting can be used as a convenient way of inserting dynamic elements into HTML templates for this assignment, similar to inline patterns found in other web languages. For more information on this technique, search the Help Desk for "quasiquote" and look at the explanations for the "," and ",@" operators. Feel free to post to the board with any questions on this (ie, the point of this assignment is not for you to spend hours figuring out this mechanism -- feel free to post sample code with questions as long as that code isn't from your servlet).

How to Run Scripts on the Server

On your own computer

The wording of this section denotes local file paths using "\" slashes as in Windows. The PLT Scheme Web Server works on multiple platforms, so you will need to change the direction of slashes if your OS requires slashes in a different direction for local paths. The PLT directory in these directions refers to the directory where you have installed Scheme on your machine.

To start the server on your home machine, run PLT\plt-web-server.exe -p <port> . A typical port is 8080.

To see some sample servlet code, look at the files in PLT\collects\web-server\default-web-root\servlets\examples .

To add a servlet to the default server, save your scheme file with a .ss extension and place it in PLT\collects\web-server\default-web-root\servlets. Note that your file must have the same base name as given in the module declaration in the file.

To run servlet PLT\collects\web-server\default-web-root\servlets\<path>\<servlet-name.ss>, go to http://localhost:<port>/servlets/<path>/<servlet-name.ss>.

If you change your servlet while the server is running, you need to tell the server to clear the old version from its cache. Do this by loading the page http://localhost:<port>/conf/refresh-servlets. I find it handy to keep this page up in another browser tab/window: hit reload on this page every time you edit the servlet, then reload the servlet.

After reloading, you will need to start the servlet again from its original URL. If the URL showing in your browser has a key of letters and numbers, you are running a continuation, which will refer to a previous version of your script. Always start new runs from http://localhost:<port>/servlets/<path>/<servlet-name.ss>.

Documentation is also available at http://localhost:<port>/defaults

On CCC Unix:

Download the install-ws.sh script to the directory where you want to put your script files, make it executable, and run it.

The produced message gives the command needed to start the server. Take your port number from this list (to make sure you don't clash with another student's port). The command is also in the script file, should you need to find it later. Running the script again will delete any scripts you installed, so please don't run it more than once!.

Once the server is running, your file will be at a URL like the following: http://ccc.wpi.edu:8087/servlets/kathi-add-servlet.ss (where I used port 8087 and put my file in default-web-root/servlets from the directory where I ran the install script).

Please kill your server process when you are done working for a session so we don't overload the CCC machines.

Debugging

If you are writing your servlet in DrScheme, executing it within DrScheme will check for compile-time errors such as unbound variable names. You can't run the script from within DrScheme (it expects the web interface). If your script generates a run-time error, your browser will show the message "servlet didn't load". You can then find the contents of the error message in Standard Out (the shell window in which you started the server or a separate window if you ran the server by double-clicking the web-server executable icon).

What to Turn In

Submit a single web script file called blog.ss. This file should be self-contained and should run as a Scheme servlet. Make sure your name is at the top of your file (in a comment).

FAQ

Do we need to include the "clear" button shown in the handout?
No, no clear button is required.

Do we need to store the blog posts across server sessions?
No. Your blog posts do not need to persist across web server sessions (ie, if the server dies, the previous posts may reset to empty). The point of this assignment is in seeing how continuations form the basis of web programming. Techniques for making data persist are orthogonal, so we'll ignore them for this assignment. A simple list of the existing posts will suffice.

Can we use a mix of Scheme and other languages?
No. All of your code for this assignment should be in Scheme,

Do we have to be able to clone windows, etc?
Cloning is an issue of handling state properly. I expect that you'll maintain the accepted posts as a list within your script. I believe that will support cloning, but if not, the list of posts (properly maintained) is all we're looking for.