cbiffle / remolt

Reduced Embeddable More Or Less Tcl

remolt - Reduced Embeddable More Or Less Tcl

This is a fork of Molt, a Rust implementation of a Tcl-like language. The goal of this fork is to produce a Tcl-like system that can run on microcontrollers without an operating system. In pursuit of that goal, the main changes are:

1. remolt is compatible with no_std platforms as long as the alloc crate is available.

2. Chunks of the implementation have been rewritten with a focus on reducing the size required in ROM (or, in Unix-speak, the "text size").

3. Many parts of the implementation have been made optional, controlled through Cargo features, allowing the build to be tailored for your specific requirements.

4. To improve performance on microcontrollers, several algorithms and code patterns have been rewritten to reduce intermediate allocations and copies.

In addition, in revisiting the Molt codebase from four years later, some opportunities for modernization and simplification became available:

• The system now uses the Rust 2021 edition.

• Patterns in the code that replicate common API (like unwrap_or_default or TryFrom) have been replaced with calls to the standard API.

• The "context map" facility has been removed in favor of the ability to register arbitrary Rust closures as Tcl commands.

• The codebase is now warning-free up through rustc 1.77.2, including Clippy.

The result is unfortunately not perfectly compatible with Molt -- in particular, the interface for providing custom commands is different. However, it's a lot smaller. Specifically:

• With the minimum set of changes to Molt to compile for thumbv7em-none-eabihf, a small driver program that blinks an LED from Tcl requires 198,184 bytes of ROM.

• Using remolt with all features except eval turned off, the same program requires 60,912 bytes of ROM --- smaller by 70%.

• Further turning off eval (to disable all expression parsing, making the engine behave roughly like partcl) brings it down to 49,048 bytes of ROM, but in practice you probably want expression parsing enabled.

Acknowledgements

Most of the code in this repo is from the Molt contributors. I only optimized the heck out of it.