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 Running Moby from DrScheme

To use Moby from DrScheme, create a file in the Module language, and at the top of your program, include the following language line:

  #lang planet dyoo/moby:1

followed by the program. For example, running the program:

#lang planet dyoo/moby:1

(define initial-world 0)

(js-big-bang initial-world (on-tick 1 add1))

will invoke a web browser, which should show the running program on a web page. The page should also provide links to download packages of the compiled program.

3 API

3.1 World

The Moby language supports reactive World-style programming; the reactive libraries allow one to write interactive web sites that work with the DOM as well as CSS stylesheets.

In the absence of an on-draw or on-redraw handler, js-big-bang will show the current state of the world; on transitions, the world is re-drawn to screen.

3.2 World Examples

Example 1:

The following will render the world as a paragraph of text, styled with a font-size of 30. Whenever the world is changed due to a stimulus, on-draw is called to re-draw the world.

(js-big-bang 0 ; initial world

; the dom tree renderer

(on-draw

(lambda (w)

(list (js-p '(("id" "myPara")))

(list (js-text "hello world"))))

; the css renderer

(lambda (w)

'(("myPara" ("font-size" "30"))))))

Example 2:

The following will show a logo and an input text field. Whenever the number in the text is changed, the world will reflect that change.

(define (form-value w)

(format "~a" w))

(define (update-form-value w v)

(string->number v))

(define elt

(js-bidirectional-input "text" form-value update-form-value))

(define (draw 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)

'(("aPara" ("font-size" "50px"))))

(js-big-bang 0

(on-draw draw draw-css))

Example:

The following program shows a ball falling down a scene.

; Simple falling ball example.  A red ball falls down the screen

; until hitting the bottom.

; The dimensions of the screen:

(define WIDTH 320)

(define HEIGHT 480)

; The radius of the red circle.

(define RADIUS 15)

; The world is the distance from the top of the screen.

(define INITIAL-WORLD 0)

; tick: world -> world

; Moves the ball down.

(define (tick w)

(+ w 5))

; hits-floor?: world -> boolean

; Returns true when the distance reaches the screen height.

(define (hits-floor? w)

(>= w HEIGHT))

; We have some simple test cases.

(check-expect (hits-floor? 0) false)

(check-expect (hits-floor? HEIGHT) true)

; render: world -> scene

; Produces a scene with the circle at a height described by the world.

(define (render w)

(place-image (circle RADIUS "solid" "red") (/ WIDTH 2) w

(empty-scene WIDTH HEIGHT)))

; Start up a big bang, 15 frames a second.

(js-big-bang INITIAL-WORLD

(on-tick 1/15 tick)

(on-redraw render)

(stop-when hits-floor?))

3.2.1 Types

A dom-sexp describes the structure of a web page:

dom-sexp

=

(list dom-element dom-sexp ...)

a css-sexp describes the structure of a page’s styling:

css-sexp

=

(listof (cons (or dom-element string)               (listof (list string string))))

Each of the dom-elements can take in an optional attribute list to assign to the new dom element; the common useful attribute is a key-value binding for an "id", which can be used to identify an element in the css-drawing function.

Here are the dom-element constructors.

A single text input form element allows the user to enter some value. That value is read from the interface by the refresh function that’s associated to the button.

3.2.2 Stimulus Handlers

Stimulus handlers are provided as additional arguments to a js-big-bang.

Each stimulus has an unstarred and a starred version; the starred version allows you to provide an effect-generating function. When the given stimulus emits, the old world is used to compute both the new world and the optional effect. Afterwards, each effect in the effect group is applied.

effect		=		atomic-effect
		|		(listof effect)

3.2.3 Effects

Effects allow world programs to apply side effects to the outside world. These are used in conjunction with the starred version of the stimulus handlers described above.

3.3 API Extensions

The following helper functions and forms are provided by Moby.

4 Developer details

The compiler takes a ASL program and translates it to Javascript code. Moby reimplements the ASL primitives in a Javascript runtime library that’s included with the compiled application. (See doc/moby-developer-api.txt for more details.)

To support smartphones, Moby uses a bridge library called Phonegap, which provides access to the native facilities of several cell phones. In this way, Moby should be able to support multiple platforms with a lot of code reuse. Moby handles the other libraries (tilt, location, sms, music), though with support only for the Android platforms for now.

4.1 Dependencies

Moby is mostly written in PLT Scheme, and the project sources are hosted by github.com. To develop with Moby, you will need the following:

• PLT Scheme >=4.2.2 (http://plt-scheme.org/)

• git (http://git-scm.com)

4.2 Installing from Developer Sources

4.3 Running Moby from the command line

By default, the command line utility will use the js backend.

After this, you can look at the Android emulator, which should now have the "FallingBall" application installed.

4.4 Compiler

The compiler is intentionally written in a small superset of the language ("src/compiler/lang.ss"). As a consequence, it is self hosting, and we take advantage of this to produce a running compiler on the browser. ("support/js/test/test-repl.html")

• "src/compiler/bootstrap-js-compiler.ss": compiles "beginner-to-javascript.ss" against itself to produce "support/js/compiler.js".

4.4.1 An example

Let’s see what beginner-to-javascript.ss gives us:

> (define p '((define (f x)

(* x x))

(f 3)))

> (define cp (program->compiled-program p))

program->compiled-program consumes a program – a list of s-expressions – and produces a compiled-program structure.

> cp

#<compiled-program>

The compiled program consists of a list of toplevel definitions and expressions.

> (compiled-program-defns cp)

"\nfunction f(x) { return plt.Kernel._star_([x,x]); }"

> (compiled-program-toplevel-exprs cp)

> cp

#<compiled-program>

The compiled program consists of a list of toplevel definitions and expressions.

> (compiled-program-defns cp)

"\nfunction f(x) { return plt.Kernel._star_([x,x]); }"

> (compiled-program-toplevel-exprs cp)

"(function (toplevel_dash_expression_dash_show0) { \n\ntoplevel_dash_expression_dash_show0((f((plt.types.Rational.makeInstance(3, 1))))); })"

If we want to embed the evaluation of this program in a web page, we can use the two strings above to do so. For convenience, we provide a helper function compiled-program-main that ties both the definitions and expression evaluation together.

4.5 Runtime

The Javascript program that’s emitted depends on a runtime kernel that’s currently implemented in Javascript. See the files in "support/js/runtime".

4.6 Ugly Hacks

The afterAttach() hack: one problem with excanvases in IE is that they can’t render until they’re attached to a parent node. Unfortunately, this means that we can’t render a canvas at a call to a value’s pretty-printing routine, because that value isn’t yet attached to the DOM.

5 Appendix

5.1 Bindings from ASL

The following toplevel bindings are available from Moby, and have the same meaning as in Advanced Student Language.

• *

• +

• -

• /

• <

• <=

• =

• =~

• >

• >=

• abs

• acos

• add1

• andmap

• angle

• append

• asin

• atan

• boolean=?

• boolean?

• build-list

• caaar

• caadr

• caar

• cadar

• cadddr

• caddr

• cadr

• car

• cdaar

• cdadr

• cdar

• cddar

• cdddr

• cddr

• cdr

• ceiling

• char->integer

• char-alphabetic?

• char-ci<=?

• char-ci<?

• char-ci=?

• char-ci>=?

• char-ci>?

• char-downcase

• char-lower-case?

• char-numeric?

• char-upcase

• char-upper-case?

• char-whitespace?

• char<=?

• char<?

• char=?

• char>=?

• char>?

• char?

• check-expect

• check-within

• check-error

• complex?

• conjugate

• cons

• cons?

• cos

• cosh

• current-seconds

• denominator

• e

• eighth

• empty

• empty?

• eof

• eof-object?

• eq?

• equal?

• equal~?

• eqv?

• error

• even?

• exact->inexact

• exp

• expt

• false

• false?

• fifth

• first

• floor

• foldl

• format

• fourth

• gcd

• identity

• imag-part

• inexact->exact

• inexact?

• integer->char

• integer?

• lcm

• length

• list

• list*

• list->string

• list-ref

• log

• magnitude

• make-posn

• make-string

• map

• max

• member

• memq

• memv

• min

• modulo

• negative?

• not

• null

• null?

• number->string

• number?

• numerator

• odd?

• pair?

• pi

• positive?

• posn-x

• posn-y

• posn?

• quotient

• random

• rational?

• real-part

• real?

• remainder

• rest

• reverse

• round

• second

• seventh

• sgn

• sin

• sinh

• sixth

• sqr

• sqrt

• string

• string->list

• string->number

• string->symbol

• string-append

• string-ci<=?

• string-ci<?

• string-ci=?

• string-ci>=?

• string-ci>?

• string-copy

• string-length

• string-ref

• string<=?

• string<?

• string=?

• string>=?

• string>?

• string?

• struct?

• sub1

• substring

• symbol->string

• symbol=?

• symbol?

• tan

• third

• true

• zero?

• begin

• set!

• let

• let*

• letrec

• case

• build-vector

• make-vector

• vector

• vector-length

• vector-ref

• vector-set!

• vector?

5.2 Unimplemented forms