Para

Para Lang

An optional .pts / .pjs syntax over Para Lib. Adds reactive bindings (signal / effect / ~> / ->), pipelines (|>), error chaining (..! / ..&), integer ranges, pure / memo declarators, and defer / arena blocks. Compiles to standard JavaScript at parse time.

The reactive forms desugar to imports from @para/signals; the rest desugar to plain JS. The .pts compiler today lives inside ParaBun; a standalone @para/transpile for non-ParaBun build hosts is in development.

Syntax (Para Lang)

Signals and effects

A signal declaration creates a reactive cell. Bare reads inside a tracked context (an effect, a derived, a when block, or another signal's RHS) compile to .get(). Bare writes compile to .set(). A signal whose initializer reads other signals is auto-promoted to a derived value. 

signal count = 0;
signal doubled = count * 2;       // derived; recomputes when count changes

effect { console.log(doubled); }  // runs once now, again on each change

count++;                          // count.set(count.get() + 1)

Edge-triggered handlers

A when block fires its body once on each false→true transition of the predicate. when not fires on the true→false transition. The predicate is tracked the same way an effect body is. 

signal score = 0;

when score >= 100 { unlockAchievement("century"); }
when not online   { showOfflineBanner(); }

Reactive bindings

A ~> B desugars to effect(() => { B = A; }), an assignment that stays in sync. A -> fn desugars to effect(() => { fn(A); }), a call binding. Both are shorthand for the common single-statement effect. 

signal name = "world";
name ~> document.title;          // title tracks name
name -> console.log;             // logs on every change

Ranges and pipelines

a..b is an exclusive integer range; a..=b is inclusive. The pipeline operator |> threads a value through a sequence of unary calls. 

for (const i of 0..n) work(i);
const evens = 0..=20 |> filter(i => i % 2 === 0);

const out = pixels |> map(p => p * 1.2) |> clamp(0, 255);

Error operators

..! is .catch. ..& is .finally. They chain naturally with await to flatten a try/catch block. 

const data = await fetch(url).then(r => r.json())
  ..! err => defaults
  ..& ()  => spinner.hide();

Compilation

Para files (.pts) are parsed by ParaBun's transpiler. (Mainline Bun doesn't recognize the syntax — the parser additions are part of the ParaBun fork.) The output is standard JavaScript with a handful of imports from para:* module specifiers.

On ParaBun, those specifiers resolve to built-in modules. On any other host (browser, Node, Bun, Deno, Cloudflare Workers, …) alias them to the matching @para/* npm packages in your bundler. 

// vite.config.ts
import { defineConfig } from "vite";

export default defineConfig({
  resolve: {
    alias: [{ find: /^para:(.*)$/, replacement: "@para/$1" }],
  },
});

The same one-line alias works for esbuild, webpack, and rollup. See the install guide for the variants.

$npm install @para/signals @para/parallel @para/pipeline
$parabun build src/main.pts --outdir dist/

ParaBun is required for the build step today (it owns the .pts parser). A standalone npm-installable transpiler (@para/transpile) is on the roadmap.

Examples

Three worked projects, one per host environment. Each is a complete project with file layout, source, and build commands.