LeftParen is a framework for quickly creating web apps. It runs on PLT Scheme v399 or greater.
You’ll need PLT Scheme v399 or greater installed. Note that this version is not officially released yet, so you you’ll need to get it from the pre-release download page.
Make sure that mzscheme is in your path. You should be ready to go if you can do this:
Welcome to MzScheme v3.99...
Note that the particular version of MzScheme you have is likely unimportant, as long as it’s 3.99 or higher.
Installing LeftParen proper is done with a PLaneT require. See Tutorials for an example of this. When you first issue one of these require commands, you’ll automatically download the LeftParen files to your local PLaneT cache. This can sometimes take a few moments, so be prepared to wait a bit.
We’re going to make a project called hello-world. Change to the directory that you’d like to make the project in. Then issue
% mzscheme -e '(require (planet "bootstrap.scm" ("vegashacker" "leftparen.plt" 1 0)))' project hello-world
This will create a hello-world project directory for you. In this directory you’ll find the script directory, which contains some useful scripts. We need to make these scripts executable though:
% chmod u+x hello-world/script/*
LeftParen has automatically generated everything we need to run our web app – we just need to start the server:
Web server started on port 8765
Listening on IP address: 127.0.0.1
Type stop to stop the server and exit
Type restart to restart the server
Point your browser to http://localhost:8765 and you should see a familiar greeting:
Now let’s try implementing the true "hello world" of web apps – a blog. First, execute the following commands from the directory in which you want to create your project directory:
% mzscheme -e '(require (planet "bootstrap.scm" ("vegashacker" "leftparen.plt" 1 0)))' project blogerton
% chmod u+x blogerton/script/* }
We need to register a couple of pages in our app. The index-page was already set up for you, but you’ll need to add a page to create new posts, and one to view them. Make the define-app call look like this:
(index-page (url "/"))
(create-post-page (url "/post"))
(view-post-page (url "/view/" (string-arg))))
Now we need to define those pages that we declared in app.scm.
(define-page (index-page req)
`(p ,(web-link "Create a new post" (page-url create-post-page)))
`(ul ,@(map (lambda (p) `(li ,(paint-blog-post p)))
(load-where '((type . blog-post))
#:sort-by 'created_at #:compare >))))
(define-page (create-post-page req)
(form '((title "Title" text) (body "Body" long-text))
#:init '((type . blog-post))
#:on-done (lambda (post) (redirect-to-page view-post-page (rec-id post)))))
(define-page (view-post-page req post-id)
(paint-blog-post (load-rec post-id #:ensure '((type . blog-post)))))
(define (paint-blog-post post)
`(div (h2 ,(rec-prop post 'title))
(p ,(rec-prop post 'body))))
You’re ready for launch. Start the server with
and you should have a basic blogging app, with persistent data, in 19 lines of code.
LeftParen was written by Rob Hunter, but it builds heavily on (and, in fact, often directly incorporates) the work of Untyped (instaservlet and dispatch), Jens Axel Soegaard (web.plt), and of course, PLT Scheme.