Version: 5.0.1
Moby: the Moby Scheme Compiler
1 What is Moby?
Moby is a project from the PLT Scheme team. The Moby compiler
consumes Advanced Student Language (ASL) programs that use World
primitives, and produces applications for mobile platforms. The
current prototype supports desktop browsers and smartphones.
Our long-term goal is to make Scheme the premiere reactive scripting
language for mobile phones.
Shriram Krishnamurthi presented the ideas behind Moby at ILC 2009 in
his talk The Moby Scheme Compiler for Smartphones.
2 Quick Start
Let’s create a simple application that rapidly shows an incrementing counter.
Create a file "counter.rkt" in the Module language with the
following content:
| #lang planet dyoo/moby:3:1 |
| (define initial-world 0) |
| (big-bang initial-world (on-tick add1)) |
Note that this program’s in a separate language that provides
extra functions like big-bang. This program can be
executed in Racket, although evaluation will halt on the
big-bang because it’s a function that
requires a Javascript web context.
For testing, the function
run-in-browser can be used to provide a mock
environment in your web browser:
This will bring up a web server and a browser window with the running program.
To create an Android apk package, you can use
create-android-phone-package.
Create a file called
"build-counter.rkt" with the following content:
Running this will take "counter.rkt" and compile it to an Android package
that can be installed.
Because Moby programs use the web, they can dynamically
generate DOM trees and style them with CSS, as in the examples below.
The next example renders the world as a paragraph of text, styled
with a font-size of 30. It uses draw-page and draw-css
to draw the web page.
| #lang planet dyoo/moby:3:1 |
| (define initial-world 0) |
| |
| (define (draw-html w) |
| (list (js-p '(("id" "myPara"))) |
| (list (js-text "hello world")))) |
| |
| (define (draw-css w) |
| '(("myPara" ("font-size" "30")))) |
| |
| (big-bang initial-world |
| (to-draw-page draw-html draw-css)) |
The next example shows an image and an input text field. As with the
previous example, it uses draw-html and draw-css to
construct the web page, and every time the world changes, the runtime environment
reacts by re-drawing the web page.
| #lang planet dyoo/moby:3:1 |
| (define (form-value w) |
| (format "~a" w)) |
| |
| (define (update-form-value w v) |
| (string->number v)) |
| |
| (define elt |
| (js-input "text" update-form-value '())) |
| |
| (define (draw-html w) |
| (list (js-div) |
| (list (js-img "http://plt-scheme.org/logo.png")) |
| (list elt) |
| (list (js-p '(("id" "aPara"))) |
| (list (js-text (format "~a" w)))))) |
| |
| (define (draw-css w) |
| '(("aPara" ("font-size" "50px")))) |
| |
| (big-bang 0 |
| (to-draw-page draw-html draw-css)) |
We can also use phone-specific features, such as geolocation.
The following program shows the current location.
| #lang planet dyoo/moby:3:1 |
| (require (planet dyoo/moby:3:1/phone/location)) |
| |
| (define (make-message w lat lng) |
| (format "I think I am at: ~s ~s" lat lng)) |
| |
| (big-bang "initial state" |
| (on-location-change make-message)) |
Note that it requires phone/location, one of the
modules provided by this package.
The last example is a phone mood ring called "mood-ring.rkt":
it shows a single DIV whose background color is controlled by the
phone’s orientation; it uses phone/tilt to get at the
orientation of the phone, and arbitrarily maps it to a color.
| #lang planet dyoo/moby:3:1 |
| (require (planet dyoo/moby:3:1/phone/tilt)) |
| |
| ; The world is a color. |
| (define initial-world (make-color 0 0 0)) |
| |
| ; tilt: world number number number -> world |
| ; Tilting the phone adjusts the color. |
| (define (tilt w azimuth pitch roll) |
| (make-color (scale azimuth 360) |
| (scale (+ pitch 90) 180) |
| (scale (+ roll 90) 180))) |
| |
| ; scale-azimuth: number -> number |
| ; Take a number going from 0-360 and scale it to a number between 0-255 |
| (define (scale n domain-bound) |
| (inexact->exact (floor (* (/ n domain-bound) 255)))) |
| |
| ; User interface. |
| (define view (list (js-div '((id "background"))))) |
| |
| (define (draw-html w) view) |
| |
| (define (draw-css w) |
| (list (list "background" |
| (list "background-color" |
| (format "rgb(~a, ~a, ~a)" |
| (color-red w) |
| (color-green w) |
| (color-blue w))) |
| (list "width" "300") |
| (list "height" "300")))) |
| |
| |
| |
| (big-bang initial-world |
| (on-tilt tilt) |
| (to-draw-page draw-html draw-css)) |
3 Running and packaging Android programs
Runs the given input-file in a context that provides mocks for
phone-specific behavior.
|
| input-file : path-string? |
| output-apk : path-string? |
Creates an Android phone package.
4 Phone API
4.1 Location (GPS)
| (require (planet dyoo/moby:3:1/phone/location)) |
| (on-location-change world-updater) → handler |
| world-updater : (world [latitude number] [longitude number] -> world) |
Constructs a world handler that watches for changes in the phone’s geographic location.
4.2 Motion sensors and tilt
| (require (planet dyoo/moby:3:1/phone/tilt)) |
| (on-acceleration world-updater) → handler |
| world-updater : (world [x number] [y number] [z number] -> world) |
Constructs a world handler that watches acceleration updates.
| (on-shake world-updater) → handler |
| world-updater : (world -> world) |
Constructs a world handler that watches shake events; if the phone is
shaken, the world-updater will fire off.
| (on-tilt world-updater) → handler |
| world-updater : (world [azimuth number] [pitch number] [roll number] -> world) |
Constructs a world handler that watches changes in orientation.
5 API
The language bindings of Moby language come from the js-vm
PLaneT package; please refer to the documentation of js-vm.