Schala - a programming language meta-interpreter

Schala is a Rust framework written to make it easy to create and experiment with toy programming languages. It provides a cross-language REPL and provisions for tokenizing text, parsing tokens, evaluating an abstract syntax tree, and other tasks that are common to all programming languages.

Schala is implemented as a Rust library schala-repl, which provides a function repl_main meant to be used as the equivalent of main() for library users. This function parses command-line arguments and either runs an interactive REPL or interprets a program non-interactively.

Individual programming language implementations are Rust types that implement the ProgrammingLanguageInterface trait and store whatever persistent state is relevant to that language. The ability to share state between different programming languages is in the works.

History

Schala started out life as an experiment in writing a Javascript-like programming language that would never encounter any kind of runtime value error, but rather always return null under any kind of error condition. I had seen one too many Javascript Uncaught TypeError: Cannot read property ___ of undefined messages, and I was a bit frustrated. Plus I had always wanted to write a programming langauge from scratch, and Rust is a fun language to program in. Over time I became interested in playing around with other sorts of programming languages as well, and wanted to make the process as general as possible.

The name of the project comes from Schala the Princess of Zeal from the 1995 SNES RPG Chrono Trigger. I like classic JRPGs and enjoyed the thought of creating a language name confusingly close to Scala. The naming scheme for languages implemented with the Schala meta-interpreter is Chrono Trigger characters.

Schala is incomplete alpha software and is not ready for public release.

Languages implemented using the meta-interpreter

• The eponymous Schala language is an interpreted/compiled scripting langauge, designed to be relatively simple, but with a reasonably sophisticated type system.

• Maaru was the original Schala (since renamed to free up the name Schala for the above language), a very simple dynamically-typed scripting language such that all possible runtime errors result in null rather than program failure.

• Robo is an experiment in creating a lazy, functional, strongly-typed language much like Haskell

• Rukka is a straightforward LISP implementation

Reference works

Here's a partial list of resources I've made use of in the process of learning how to write a programming language.

Type-checking

https://skillsmatter.com/skillscasts/10868-inside-the-rust-compiler https://www.youtube.com/watch?v=il3gD7XMdmA http://dev.stephendiehl.com/fun/006_hindley_milner.html https://rust-lang-nursery.github.io/rustc-guide/type-inference.html

https://eli.thegreenplace.net/2018/unification/ https://eli.thegreenplace.net/2018/type-inference/ http://smallcultfollowing.com/babysteps/blog/2017/03/25/unification-in-chalk-part-1/

Evaluation

Understanding Computation, Tom Stuart, O'Reilly 2013

Basics of Compiler Design, Torben Mogensen

Parsing

http://journal.stuffwithstuff.com/2011/03/19/pratt-parsers-expression-parsing-made-easy/ https://soc.github.io/languages/unified-condition-syntax

Crafting Interpreters

LLVM

http://blog.ulysse.io/2016/07/03/llvm-getting-started.html

###Rust resources https://thefullsnack.com/en/rust-for-the-web.html

https://rocket.rs/guide/getting-started/