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:2:7

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

#lang planet dyoo/moby:2:7

(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. Because on-tick is used, as every second passes, the runtime sends a tick stimulus to the program. The page should also provide links to download packages of the compiled program.

These programs run on the user’s web browser; they can also dynamically generate DOM trees and style them with CSS, as in the examples below.

The following example renders the world as a paragraph of text, styled with a font-size of 30. It uses draw-html and draw-css to draw the web page.

#lang planet dyoo/moby:2:7

(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"))))

(js-big-bang initial-world

(on-draw 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:2:7

(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"))))

(js-big-bang 0

(on-draw draw-html draw-css))

3 The Moby World API

By default, the page that’s displayed contains a rendering of the world value. In the presence of an on-draw or on-redraw handler, js-big-bang will show a customized view.

The majority of the handlers register different stimuli that can trigger changes to the world. One instance is on-tick, which registers a function to update the world on a clock tick.

#lang planet dyoo/moby:2:7

(define WIDTH 320)

(define HEIGHT 480)

(define RADIUS 15)

(define INITIAL-WORLD 0)

(define (tick w)

(+ w 5))

(define (hits-floor? w)

(>= w HEIGHT))

(check-expect (hits-floor? 0) false)

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

(define (render w)

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

(empty-scene WIDTH HEIGHT)))

(js-big-bang INITIAL-WORLD

(on-tick 1/15 tick)

(on-redraw render)

(stop-when hits-floor?))

3.1 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 attrib)))

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.

3.2 dom-element constructors

Here are the dom-element constructors.

The example below has a single text input form element, which allows the user to enter some value. That value is read from the interface by the refresh function that’s associated to the button.

#lang planet dyoo/moby:2:7

(define input-node

(js-input "text" '(("id" "myname"))))

(define (refresh w)

(get-input-value "myname"))

(define (draw w)

(list (js-div)

(list (js-div) (list (js-text (format "I see: ~s~n" w))))

(list (js-div) (list input-node))

(list (js-div) (list (js-button refresh)

(list (js-text "Update!"))))))

(define (draw-css w)

'())

(js-big-bang ""

(on-draw draw draw-css))

3.3 Stimulus Handlers

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

Each stimulus has an effect-less and an effect-full version; the effect-full version allows you to provide an effect-generating function as well as a world-updater. 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)

3.4 Effects

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

3.5 API Extensions

The following helper functions and forms are provided by Moby.

Note: this feature is experimental and will be replaced in the near future.

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

• git

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!

• for-each

• printf

• let

• let*

• letrec

• case

• build-vector

• make-vector

• vector

• vector-length

• vector-ref

• vector-set!

• vector?

5.2 Unimplemented forms