Banish – A declarative DSL for rule-based state machines in Rust

Name: Banish – A declarative DSL for rule-based state machines in Rust
Availability: InStock
Author: LoganFlaherty

by LoganFlaherty·Feb 19, 2026·2 points·0 comments

Visit Project View on HN

AI Analysis

●●●BangerBig BrainWizardry

Fixed-point rule evaluation as a procedural macro; compiles to zero-overhead Rust, not a VM.

Strengths

•Zero runtime overhead: macro expands to plain Rust, no interpreter or virtual machine penalty
•Fixed-point solving elegantly expresses constraint/layout problems that normally need nested loops
•Scope awareness lets you reference outer variables/functions directly; seamless integration into existing code

Weaknesses

•Audience is niche: only relevant for devs building specific patterns (layout, constraint, game engines)
•Minimal real-world examples shown; unclear how complex multi-domain logic scales vs handwritten code

Post Description

I’ve been working on a Rust macro called Banish for defining rule-based state machines declaratively.

Instead of manually writing transition loops, Banish evaluates rules within a state until no rule fires (a fixed-point model), then transitions. The macro expands into regular Rust, which allows for seamless integration.

The goal is to make complex rule logic and state machines easier to express while keeping runtime costs identical to handwritten code.

The project was featured as Crate of the Week in the Rust newsletter this week. I'd love to hear your feedback.

Example: use banish::banish;

fn main() { let buffer = ["No".to_string(), "hey".to_string()]; let target = "hey".to_string(); let idx = find_index(&buffer, &target); print!("{:?}", idx) }

fn find_index(buffer: &[String], target: &str) -> Option<usize> { let mut idx = 0; banish! { @search // This must be first to prevent out-of-bounds panic below. not_found ? idx >= buffer.len() { return None; }

found ? buffer[idx] != target { idx += 1; } !? { return Some(idx); } // Rule triggered so we re-evalutate rules in search. } }