An implementation of a tiny Scheme subset

This is an implementation of a (currently miniscule) subset of R7RS-small. See the examples directory and the smoke testsfor to get an idea of what already works.

The architecture is currently a fairly basic interpreter, with syntax analysis phase up-front, which will transform the raw S-expressions into an AST, which is then evaluated. This code sprang from code intended as an example for using the lexpr crate to implement a simple S-expression calculator, and got a bit out of hand 😀.

The current architecture is insufficient to support several major language features of Scheme, such as macros or continuations. The plan is to gradually improve the implementation to make such features possible, before investing in additional features based on the current architecture.

The evolution of poco-scheme will probably be largely guided by the chibi-scheme codebase. chibi-scheme is a full implementation of R7RS-small, written in C, employing a bytecode VM, and using syntactic closures as the basic macro mechanism.

Features already present

• Proper tail calls.
• Garbage collection employing the gc crate.
• define, lambda, if.
• Fixnums, and some procedures working on numbers.
• A very crude version of display and newline, both without port arguments.

Next steps

• Basic cons cell support.
• Implement more syntax in lexpr:
  • Quote syntactic shorthand (i.e. ').
• Add a transformation of the AST to some kind of bytecode.