# 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

### 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
http://www.lihaoyi.com/post/ZeroOverheadTreeProcessingwiththeVisitorPattern.html?a=1

[Crafting Interpreters](http://www.craftinginterpreters.com/)

### 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/