# 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 common REPL, and a trait `ProgrammingLanguage` with 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_lib`, which provides a
`schala_main` function. This function serves as the main loop of the REPL, if run
interactively, or otherwise reads and interprets programming language source
files. It expects as input a vector of `PLIGenerator`, which is a type representing
a closure that returns a boxed trait object that implements the `ProgrammingLanguage` trait,
and stores any persistent state relevant to that programming language. The ability
to share state between different programming languages is in the works.

## About

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

### 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/

[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/