Bus Scheme

    by Phil Hagelberg (c) 2007 - 2008
    http://bus-scheme.rubyforge.org

Description

Bus Scheme is a Scheme written in Ruby, but implemented on the bus! Every programmer must implement Scheme as a rite of passage; this is mine. Note that at least half of the implementation of Bus Scheme must be written while on a bus. Documentation, tests, and administrivia may be accomplished elsewhere, but the majority of actual implementation code is strictly bus-driven. Bus Scheme is primarily a toy; using it for anything serious is (right now) ill-advised.

Bus Scheme aims for general Scheme usefulness optimized for learning and fun. It’s loosely targeting R5RS, but varies in huge ways. (For the purposes of this project we pretend that R6RS never happened.) See the file R5RS.diff for an incomplete list of ways in which Bus Scheme differs from the standard, both things that are yet unimplemented and things that are intentionally different.

Usage

  $ bus # drop into the REPL

  $ bus -e "(do some stuff)"

  $ bus foo.scm # load a file

Tutorial

See http://technomancy.us/104 for a "Getting Started" tutorial.

Contributing

If you’re looking for stuff to do, try "rake todo"

Patches are welcome especially if they were written while riding a bus. If your daily commute does not involve a bus but you want to submit a patch, we may be able to work something out regarding code written on trains, ferries, or perhaps even carpool lanes.

Join the mailing list to ask questions and discuss: http://rubyforge.org/mail/?group_id=5094

What makes Bus Scheme different?

Well, for starters it’s implemented on the bus. No other Scheme implementation can claim this. Here are a few other things that set Bus Scheme apart:

Flexible calling syntax

Taking a hint from Arc, Bus Scheme allows you to use the notation (mylist n) to access the nth place of the mylist list instead of (nth mylist n) or the (myhash key) notation to access the slot in myhash corresponding to the value of key instead of (gethash myhash key). TODO: This notation is flexible, and other data types may have their own "call behaviour" specified.

Web functionality

TODO: Web and RESTful application development are part of the package. Bus Scheme uses the Rack library to allow scheme programs to serve web applications. Representations of data can be easily translated between s-expressions, HTML, and JSON.

Written in a high-level language

Bus Scheme is written in Ruby, which means anyone with experience in high-level dynamic languages (like, oh, I don’t know… Scheme?) should be right at home poking around at the implementation. Using Ruby allows the implementation code to remain compact and concise. Bus Scheme should run on Ruby 1.8, Ruby 1.9, Rubinius, and JRuby at least. Bus Scheme also allows you to drop into Ruby when that’s convenient. TODO: allow real inline Ruby blocks instead of access via a function call.

Test-Driven

Bus Scheme is written in an entirely test-driven manner. As much as possible, it tries to keep its tests written in Scheme itself, so it includes a fairly comprehensive testing suite and encourages programs to be written test-first.

Install

• sudo gem install bus-scheme

For the source:

• git clone git://github.com/technomancy/bus-scheme.git

Todo

Bus Scheme is currently missing pieces of functionality:

Parser

• Single-quote doesn’t always work like (quote […])
• multiline strings
• regular expressions

General

• filter stacktrace
• continuations
• syntax rules

Failing tests for some of these are already included (commented out, mostly) in the relevant test files.

Long Term (post 1.0)

• web functions (defresource and friends)
• (lambda (arg1 arg2 . args) body) for rest args
• string interpolation
• escape sequences in strings
• Ruby blocks inline?
• optimize tail call recursion
• compile to Rubinius bytecode
• custom call behaviour
• parse non-decimal base numbers
• parse rationals, scientific, complex, and polar complex numbers

Requirements

Bus Scheme should run on (at least) Ruby 1.8, Ruby 1.9, Rubinius, JRuby. Any support for Windows is entirely accidental.

Bonus Fact

I haven’t actually used a real Scheme yet. Everything I know about it I’ve gathered from reading The Little Schemer, watching the Structure and Interpretation of Computer Programs videos, reading lots about Common Lisp, and writing lots of Emacs Lisp. If there are huge gaping flaws in the implementation, this is likely to be why.