I show how powerful incremental development with Lisp/Scheme is by showing a website and game I created by running the application and developing them real-time.

1. INCREMENTAL DEVELOPMENT WITH LISP
Building a Game and a Website
@jlongster
James Long

2. INCREMENTAL DEVELOPMENT
• “acyclic software development process developed in
response to the weaknesses of the waterfall model”
- Wikipedia
• Rapidly evolving software by means of frequent small changes

3. CODING
• Let’s
focus on PROGRAMMING, apply the same model to just
the programmer
• Write small pieces of code, run them, and repeat
• Tweak and extend the program while it is running
• Advantages: instant results, higher morale, less rabbit trails

4. WHY LISP?
• Lisp is famous for embracing incremental development
• Very expressive and dynamic
• Functional
simplicity allows for “piece-by-piece” evaluation, or
running small chunks of code quickly
• Thephilosophy of Lisp is built around incremental
development

5. IN PRACTICE
• How do you actually write code incrementally?
• REPL
• IDE
• Emacs
• SLIME
• comint-mode

6. EXAMPLE #1
VECTORS
• Goal: Write a 3d vector library with support for addition
(define-type v3
x y z)
EVAL
(define (v3-add v1 v2)
(make-v3
(+ (v3-x v1) (v3-x v2))
(+ (v3-y v1) (v3-y v2))
(+ (v3-z v1) (v3-z v2))))
EVAL
• Intimate with what’s going on with the code

7. EXAMPLE #1
VECTORS
• Goal: Write a 3d vector library with support for addition
(define-type v3
(define-type v3 constructor: really-make-vec3d
x y z) x y z)
EVAL EVAL
(define (v3-add v1 v2) (define (make-v3 x y z #!key (scale 1.))
(make-v3 (really-make-vec3d (* x scale)
(+ (v3-x v1) (v3-x v2)) (* y scale)
(+ (v3-y v1) (v3-y v2)) (* z scale)))
(+ (v3-z v1) (v3-z v2))))
EVAL
EVAL
v3-add is still available
• Intimate with what’s going on with the code

8. EXAMPLE #2
RENDER QUEUE
• Goal: Write a library which queues objects to render
(define queue '())
(define (queue-add obj)
(set! queue (cons obj queue)))
(define (queue-render-all)
(for-each (lambda (obj)
(write obj))
queue))
EVAL-REGION
• Modify a program while its running and never throw away
state

9. EXAMPLE #2
RENDER QUEUE
• Goal: Write a library which queues objects to render
(define queue '())
(define (queue-render-all)
(define (queue-add obj) (display "--- RENDERED OBJECTS ---n")
(set! queue (cons obj queue))) (for-each (lambda (obj)
(display obj)
(define (queue-render-all) (newline))
(for-each (lambda (obj) queue))
(write obj))
queue)) queue and queue-add are the same as before
EVAL-REGION
• Modify a program while its running and never throw away
state

10. INITIAL THOUGHTS?
• Emacs greatly enhances the ability for “live coding”, could be
done in other editors though
• “Scratch buffers” are commonly used to test the system
• Your “scratch code” evolves into unit tests

11. REAL WORLD EXAMPLE #1
WEBSITE
• Clojure is a Lisp dialect which runs on the JVM
• Compojure is a web framework developed in Clojure
• jlongster.com
• Clojure
supports SLIME, an incredible development
environment fully embracing incremental development

12. REAL WORLD EXAMPLE #2
GAME
•Iused Gambit Scheme to make
an iPhone game, Farmageddon
• Wrote a tool that embeds
“remote REPLs” in applications
so that you can develop it
SLIME-style
• Extended Emacs keybindings

13. REAL WORLD EXAMPLE #2
GAME
•Iused Gambit Scheme to make
an iPhone game, Farmageddon
• Wrote a tool that embeds
“remote REPLs” in applications
so that you can develop it
SLIME-style
• Extended Emacs keybindings

14. PROBLEMS
• Youmust write your code in a way that allows incremental
development
• Your IDE must support it
• Sometimes there’s a lot of overhead for little gain

15. WHAT ABOUT YOU?
• Most of you don’t use Lisp, probably
• Python and many other languages could support this too
• It’s a different mindset that I challenge you to try

16. • Buymy game, available next week on the App Store for only
$1.99
• @farmageddongame
• http://farmageddongame.com