ΛLogos

Writ: code + data

Writ is what makes Logos more than a Rust-flavored systems language. It’s a relocatable, schema-aware, tagged data substrate — and it is built into the language, not bolted on as a library, DSL, or macro system.

Why it’s part of the language

In most languages, structured data (JSON, protobufs, config trees) lives on the far side of a boundary: you parse it into values, mutate values, then serialize back. Writ erases that boundary.

  • @{…} and @[…] are literal forms in the grammar — object and array Writ literals, right in your source.
  • Capture — $ident and ${expr} — splices Logos values into a Writ literal and is type-checked at sema time.
  • View types carry lifetimes through the borrow checker, so borrowing into a Writ graph is as safe as borrowing into any other value.
  • Module-scope literals fold to rodata — constant Writ documents become read-only data in the binary, with no runtime construction cost.

No DSL. No macros. No FFI between values and data.

Round-tripping a document

Parsing and re-rendering a Writ document is a call each — the parsed graph is a first-class, owned value:

package writ_example;
use logos.lang.writ.container;   // Writ
use logos.mem.writ.parser;       // parse_writ
use logos.mem.writ.stringify;    // stringify
use logos.std.io;

fn main() -> i32 {
    let doc: Writ = parse_writ(r#"
        {
            name:"widget",
            version:42,
            active:true,
            tags:["fast","safe"],
            i32_array: <I32> [1,2,3,4]
        }
    "#);
    if doc.root().is_null() { return 1; }   // null root == parse error
    let s: String = stringify(doc.root());
    println(s.as_str());
    return 0;
}

Note the <I32> [1,2,3,4] form — Writ is tagged and schema-aware, so element types travel with the data.

Where Writ fits

Writ underpins a lot of Logos: metaprogramming hooks synthesize source as Writ, the runtime models Datatype / Storage / View with zones and a type registry, and compile-time programming is ordinary Logos code operating over Writ graphs.

For the full runtime model, see the in-repo docs under docs/internals/writ-runtime.md in the Logos repository.