An Introduction to Web Development with PLT Scheme

Introduction

From time to time people ask how to develop for the web with PLT Scheme on the PLT mailing list. The quick answer is "Just as in any other language", but that's not how to get people hooked on Scheme. To write a decent web-application require knowledge of a range of subjects such as HTML, databases, servlets, and web-servers. For some reason there is a lack of tutorials on these subjects, so I have decided to make an attempt at writing, if not a complete tutorial, then an elaborate get-started example.

The example application will be a mini version of Reddit called ListIt. The front page consists of a list of links to interesting articles, users can vote the articles up and down, and submit new articles. The hope is that the example is small enough to be easily understood, but on the other hand large enough to illustrate as many aspects as possible. Please leave comments on the blog: Did the example hit home? Is a paragraph in need of a rewrite? Did I skip something?

Model-View-controller

First things first. How should the program be organized? There is no need to reinvent the wheel, so I have chosen to use the Model-View-Controller architecture, which works just as well for web applications as it does for graphical user interfaces.

In a nutshell the Model-View-Controller architecture works like this: The model holds the data, the view displays data. User interactions goes through the controller in order to keep a separation between the model and the view. (See section 22.3 of HTDP or Wikipedia for more on MVC ).

In our case we will represent the model, the view and the controllers as three separate Scheme modules. The model will use a database to hold the links, the view will consists of functions generating HTML and the controller will the web-servlet that reacts on the user actions.

The Model

Today we will concentrate on the model. Each entry in our database consists of an entry-id , a title to display, an url to the article and a score representing the votes. Since we expect many entries in our database, we will think of them as divided into page. The number of entries in each page is given by the parameter PAGE-LIMIT.

The interface to our model consists of the following functions:

insert-entry: title url score -> entry-id
Insert a new entry into the database.

increase-score : entry-id ->
Increase the score of an existing entry

decrease-score : entry-id ->
Decrease the score of an existing entry

top : natural -> (list (list entry-id title url score))
Return the given number of entries with the highest scores

page : natural -> (list (list entry-id title url score))
Return the list of entries in the given page.

url-in-db? : url -> boolean
Is the url already listed?

These functions are the only ones to be exposed to the controller.

Implementation of the model

To implement these functions we will use an SQLite database. It wouldn't be unreasonable to argue that it would be easier to use a hash-table, but I want to illustrate how to use SQLite.

SQLite is small database engine, which comes in the form of a single self-contained, zero-configuration DLL-file on Windows or a a so-file on other platforms. We will use Jay McCarthy and Noel Welsh's PLT Scheme bindings sqlite.plt . On top of these binding we'll use a S-expression to SQL-string library written by me (it will appear on PLaneT soon - it has been submitted). On Windows you download SQLite by pasting the following into the DrScheme interaction window (the REPL):

(require (planet "download-sqlite.scm"
("soegaard" "sqlite.plt" 1 0))

To use the two SQLite packages, we start our module with

(require
(planet "sqlite.ss" ("jaymccarthy" "sqlite.plt"))
(planet "sqlite.ss" ("soegaard" "sqlite.plt" 1 0)))

Opening the database is simple:

(define db (open (string->path "c:/listit.sb"))) .

At least it will be, after it is created. The following function creates an empty database with a single table "entries":

(define (create-table-entries)
 (exec/ignore
  db
  #<<SQL
CREATE TABLE entries (
  entry_id  INTEGER PRIMARY KEY,
  title     TEXT,
  url       TEXT,
  score     INTEGER )
SQL
  ))

The #<< starts a so called here-string. The function exec/ignore executes an SQL-statement and ignores the result (there is no S-expression syntax for CREATE TABLE yet).

Once we have created the our table, we can begin writing the functions in our interface.
The first is:

(define (insert-entry title url score)
 (insert db (sql (INSERT INTO entries (title url score)
                         VALUES (,title ,url ,score)))))

The macro sql converts an S-expression representation of an SQL-statement into a string, which is then handed to SQLite by insert. The string produced by the sql macro from
(insert-entry "Everything Scheme" "http://www.scheme.dk/blog/" 42)
becomes
"INSERT INTO entries (title, url, score) VALUES ('Everything Scheme', 'http://www.scheme.dk/blog/', '42')".

The remaining functions from the interface are all simple SQL-statements, which can be studied in the full program below.

A loose end: In the source below the parameter current-database is used to hold the database. As a convenience I have with the help of syntax-id-rules defined the identifier db to expand to (current-database). But in order make everything work also as when the database isn't created yet, the actual definition below is a little more involved.

Testing

To test the model, open the "model.scm" in DrScheme. In the "Language" menu use "Choose Language" to choose the "Module" language. Click and "Run" and you are ready to test it:

Welcome to DrScheme, version 369.3-svn10jan2007.
Language: (module ...).
> (insert-entry "Everything Scheme" "http://www.scheme.dk/blog/" 42)
1
> (insert-entry "Reddit" "http://www.reddit.com" 7)
2
> (insert-entry "PLT Scheme" "http://www.plt-scheme.org" 5)
3
> (top 2)
(#4("entry_id" "title" "url" "score")
#4("1" "Everything Scheme" "http://www.scheme.dk/blog/" "42")
#4("2" "Reddit" "http://www.reddit.com" "7"))
> (increase-score 3)
> (increase-score 3)
> (increase-score 3)
> (increase-score 3)
> (top 2)
(#4("entry_id" "title" "url" "score")
#4("1" "Everything Scheme" "http://www.scheme.dk/blog/" "42")
#4("3" "PLT Scheme" "http://www.plt-scheme.org" "9"))
> 

The Program

;;; model.scm  -- Jens Axel Søgaard

(module model mzscheme
(provide (all-defined))

(require
 (planet "sqlite.ss" ("jaymccarthy" "sqlite.plt"))
 (planet "sqlite.ss" ("soegaard" "sqlite.plt" 1 0)))

;;; CONFIGURATION

(define PAGE-LIMIT        (make-parameter 50))   ; number of entries on each page
  (define DATABASE-PATH     (string->path "listit.db"))
; initialization of the db happens at the first run, see bottom of this file
  (define current-database  (make-parameter #f))

;;; CONVENIENCE

; define db to be short for (current-database)
  (define-syntax db
  (syntax-id-rules () [db (or (current-database)
                              (let ([d (open DATABASE-PATH)])
                                (current-database d)
                                d))]))

;;; DATABASE CREATION

(define (create-table-entries)
  (exec/ignore
   db
   #<<SQL
 CREATE TABLE entries (
   entry_id  INTEGER PRIMARY KEY,
   title     TEXT,
   url       TEXT,
   score     INTEGER )
SQL
   ))

(define (drop-table-entries)
  (exec/ignore db "DROP TABLE entries"))


;; DATABASE INSERTION AND UPDATES

(define (insert-entry title url score)
  (insert db (sql (INSERT INTO entries (title url score)
                          VALUES (,title ,url ,score)))))

(define (increase-score entry-id)
  (update db (sql (UPDATE entries
                          SET (score = (+ score 1))
                          WHERE (= entry_id ,entry-id)))))

(define (decrease-score entry-id)
  (update db (sql (UPDATE entries
                          SET (score = (- score 1))
                          WHERE (= entry_id ,entry-id)))))


;;; DATABASE RETRIEVAL

(define (top n)
  (select db (sql (SELECT (entry_id title url score)
                          FROM entries
                          ORDER-BY (score DESC)
                          LIMIT ,n))))

(define (page n)
  (select db (sql (SELECT (entry_id title url score)
                          FROM entries
                          ORDER-BY (score DESC)
                          LIMIT ,(PAGE-LIMIT) OFFSET ,(* (PAGE-LIMIT) n)))))

(define (entries-with-url url-str)
  (select db (sql (SELECT (entry_id title url score)
                          FROM entries
                          WHERE ,(format "url='~a'" url-str)))))

(define (url-in-db? url-str)
  (let ([result (entries-with-url db url-str)])
    (if (null? (entries-with-url db url-str))
        #f
        result)))


;;; INITIALIZATION

; on first run, create tables
  (unless (and (file-exists? DATABASE-PATH)
             (table-exists? db "'entries'"))
  (create-table-entries)
  (current-database (make-parameter (open DATABASE-PATH)))))