Liquid Lox

This is an interpreter for the Lox language from Crafting Interpreters. Lox is a dynamic, object-oriented language loosely based on JavaScript. See the language overview for features of the language.

A few notable highlights of this implementation:

• Passing the reference implementation's test suite
• Arrays
• Metaclasses
• Unicode variable names
• Written in 100% safe Rust
• Free software license

A second entire implementation using a byte-code virtual machine is under development.

See below for the gory details.

Tree-Walking Interpreter

This project contains two separate implementations of the language.

The first implementation, the tree-walking interpeter, is feature-complete and is designed to be simple for experimentation.

Differences from the Reference Implementation jlox

Optional challenges and features implemented in the tree-walking interpreter:

• Pass the reference implementation's test suite (see status for exact counts)
• Column number of source, in addition to line number, is displayed in error messages
• Plus operator coerces to string when one value is a string
• Division by zero is a runtime error
• REPL accepts expressions and statements
• break and continue statements inside loops
• anonymous function expressions
• Look up variables by index, instead of by name, which is much faster
• Static class methods that can access this
• Get and set properties on classes using metaclasses

Custom features not mentioned in the book:

• Unicode variable names using grapheme clusters, not just code points
• Arrays
  • literals [1, 2, 3]
  • get arr[index]
  • set arr[index] = exp
  • Native function: array_create(length, fill_value)
  • Native function: array_length(arr)
  • Native function: array_push(arr, item)
  • Native function: array_pop(arr)

Reference Tests

To ensure correctness, the official test suite is run.

Current status:

• 224 tests passed
• 15 tests failed
• 8 failures are due to optional features and are false positives
• 7 failures are minor differences in parse error messages

Like the reference implementation, we skip limit tests that don't apply.

Known Issues

• Because the interpreter uses reference counting and doesn't (yet) implement a garbage collector, many input programs create reference cycles and leak memory.

Bytecode VM

The bytecode virtual machine is the second implementation in this project and is currently under development. Major features that haven't been done yet are:

• functions
• closures
• classes
• garbage collection

Differences from the Reference Implementation clox

Optional features implemented in the bytecode VM:

• break and continue statements inside loops

Custom features not mentioned in the book:

• Unicode variable names using grapheme clusters, not just code points

Building

cargo build --release

The result will be ./target/release/lox.

Usage

Run a Lox script:

lox script.lox

Run a Lox REPL:

lox

To use the byte-code virtual machine, which is still a work-in-progress, use:

lox --vm

Running Tests

cargo test

Note: This runs the internal test suite. The reference tests aren't included in this repo.