A toy example of a recursive descent parser and lexer for a small arithmetic expressions language written in Rust. The parser builds a syntax tree and features a simple evaluator that computes the expressions's value by traversing a syntax tree. Example expression: 0 + 8 * 15. I created this to satisfy my curiosity about the language.

The tokens that the lexer needs to recognize (modulo whitespace).

MulOp := '*' | '/'
AddOp := '+' | '-'
Num := ['0' - '9']+

This is the base grammar.

Start → Exp
Exp → Exp + Term
Exp → Exp – Term
Exp → Term
Term → Term ∗ Num
Term → Term / Num
Term → Num

We cannot use left-recursive grammars to build a recursive descent parser. So we just can transform the grammar as follows:

Start → Exp
Exp → Term Exp′
Exp′ → '–' Term Exp′
Exp′ → '+' Term Exp′
Exp′ → 𝜖
Term → Num Term′
Term′ → '∗' Num Term′
Term′ → '/' Num Term′
Term′ → 𝜖 

This is the grammar that was used to create the toy example.

Install Rust and Cargo on your system as described here. Clone this repo, cd to the checkout directory and run cargo test. This should just compile the sources and run the accompanied unit tests.

For license information just read the license file.