zia: Interpreter for the Zia programming language.

The Zia project aims to develop a programming language that can be used to program itself. Instead of storing the source code as plain text and editing the raw text (which can easily break the program), the runtime environment of the interpreter (the Context) can be saved to disk and used in other programs. All the programming is done using an interactive shell such as IZia. The commands sent are interpreted based on the Context. They are used to incrementally modify, test and debug the Context.

Expressions for Zia commands represent a binary tree where parentheses group a pair of expressions and a space separates a pair of expressions.

e.g. (ll lr) (rl rr)
represents the following binary tree: / \ / \ / \ / \ / \ ll lr rl rr

The leaves of the tree can be any unicode string without spaces or parentheses. These symbols may be recognised by the intepreter as concepts or if not used to label new concepts.

Currently, only the lowest-level functionality has been implemented. It's important that programs are represented consistently and transparently within the Context in order to achieve a self-describing system. The syntax shown below may appear awkward but more convenient syntax will be possible once more functionality is added. For example, the need to group pairs of expressions in parentheses will be alleviated by functionality to set the relative precedence and associativity of concepts.

So far there are 4 built-in concepts. A new Context labels these with the symbols below but the labels can be changed to different symbols (e.g. for different languages or disciplines).

Let concept, symbol: let

Label concept, symbol: label_of

Reduction concept, symbol: ->

Definition concept, symbol: :=

Public API

The current implementation exposes the Context type that can be used in an interface such as IZia. Importing the following traits allows the corresponding methods to be called with Context.

```rust trait ContextMaker { fn new() -> Self { // Constructs a new Context and labels the 4 built-in concepts. } }

trait Execute { fn execute(&mut self, command: &str) -> String { // Executes the commands given by the user that may modify the Context and outputs a // String that maybe empty, an error message or the answer to a query. } } ```