fable

fable

fable is a tiny typed language for inspecting and mutating facet-reflected Rust values, then lowering toward canonical Weavy IR. You write a small script that navigates a struct by field name and assigns or tests scalar values; fable resolves every path against the Facet shape at compile time, type-checks the expressions, and produces a reusable bytecode plan that runs directly against a live &mut T without any serialization round-trip.

The crate currently owns the syntax layer — a lossless lexer/parser, the cstree language tags, and a typed AST facade — plus the lowering and interpreter that compiles source to Weavy IR and executes it against Facet-reflected Rust values. "Lossless" means the concrete syntax tree preserves every byte — whitespace, comments, trivia — so tooling can round-trip and rewrite Fable source without losing anything.

rust
use facet::Facet;
use fable::{FablePlan, apply};

#[derive(Facet)]
struct Config {
    name: String,
    max_retries: u32,
    threshold: f64,
}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut cfg = Config {
        name: "  acme  ".into(),
        max_retries: 100,
        threshold: 0.5,
    };

    // Compile once, apply many times.
    let plan = FablePlan::<Config>::compile(
        r#"
        root.name = trim(root.name);
        root.max_retries = clamp(root.max_retries, 1, 10);
        if root.threshold > 1.0 {
            root.threshold = 1.0;
        }
        "#,
    )?;
    plan.apply(&mut cfg)?;

    // One-shot convenience wrapper.
    apply(&mut cfg, r#"root.name = root.name + "-v2";"#)?;

    Ok(())
}

How it fits with Facet and Weavy

Fable sits between the two: it reads struct layouts from facet-core reflection (Shape, field offsets, scalar types) and emits Weavy IR that the Weavy interpreter executes. Because field paths are resolved at FablePlan::compile time, repeated apply calls pay only interpreter cost — no parsing, no field lookup.

Supported scalar types cover the full Rust numeric tower (i8–i128, u8–u128, f32, f64), bool, char, String, and Cow<str>. Built-in intrinsics include min, max, clamp, len, trim, contains, starts_with, and ends_with.

License

Licensed under either of:

at your option.