zaoqi-unsafe / scheme.lua

Simple Scheme interpreter written in Lua

Scheme.Lua

This is a Scheme dialect compiler for Lua.

It parses S-expressions into Lua structures and evaluates them.

You will need Lua 5.1 or 5.2 to use it.

You can run an interpreter to evaluate any Scheme file (well not any, only subset of Scheme is supported for now, but nevertheless):

$ ./bin/runscm.lua code.scm

You can "compile" Scheme code into Lua data structures representation to speed up code loading:

$ ./bin/compile.lua code.scm
$ chmod a+x code.lscm

This will create code.lscm file by default, where lscm is for Lua Scheme.

Also there's simple REPL in example dir:

$ cd example
$ ./repl.scm

This project is a work in progress, so it's buggy and a lot of things missing.

Disclaimer

Although I have good experience in Lua, I'm just a novice in Scheme/Lisp.

This whole project is made to learn Scheme (which I like more than Common Lisp), and I learn as I write it, so if you are a Lisp or Scheme expert, don't blame me too much if I implement language features incorrectly. Just open an issue in this repo or submit a pull request with corrections so I can fix things up. I'm happy to learn from you!

Not implemented

• (call/cc) and continuations, and it's unlikely I implement them as it's non-trivial to do it in Lua, maybe I will implement (call/cc1) at some point (and there's a strong chance it will be based in coroutines).

  Anyway you can use coroutine standard library from Scheme land by importing it with (lua-import coroutine) form.

• Error handling with line numbers and full stack backtrace reports.

  Most errors are propagated to Lua land as is with thin wrapper around error() function, and you can deduct what's wrong from normal Lua stack backtrace, Lua functions (which back basic Scheme functionality) being named by file/line pairs, but no try is made to guess the error location in source Scheme code, which is sad and I know it.

  I leave it as is now due to performance restrictions and because usage of pcall() breaks tail recursion chain, and this makes whole thing blow up as all the evaluation is based in Lua's tail recursion mechanism.

  If I think of any way to handle errors more Scheme-way without serious performance impact and without killing code evaluation with stack overflows, I will do it.

• Scheme macro system and syntax extension forms.

  I don't plan to implement it in near future, as the Scheme dialect is easily extensible with Lua code. You can always write your own syntax structure in Lua and plug it into Scheme land.

• A huge number of number types available in Scheme (no pun intended).

  Scheme supports exact and inexact numbers, as well as integers, reals, complex numbers etc. Lua only has floats (or integers, depending on compilation flags). I try to parse most simple number literals in Scheme sources, but all numbers end up as simple Lua numbers, and it's unlikely to change in near future as implementing it in Lua will be very cumbersome and slow. So don't expect great precision from your "exact" numbers. You were warned!

• A lot of Scheme forms, I will implement at some point (except for things which are too difficult or impossible to do in Lua).

Contributions

Of couse any contribution is welcome! Fork me, create topic branch, do your fancy things and submit a pull request to me to merge.

Thank you!

© 2013 Konstantin Stepnov