MEL Domain Basics

A MEL domain names your state, derived values, and allowed actions.

MEL is the authoring format for Manifesto domains. It compiles to data the runtime can run the same way each time.

A Small Domain

domain Counter {
  type Label = {
    text: string
  }

  state {
    count: number = 0
    label: Label = { text: "Counter" }
  }

  computed doubled = count * 2

  action increment() {
    onceIntent {
      patch count = count + 1
    }
  }
}

What Each Part Means

Part	Purpose
domain Counter	Names the domain model
type Label	Defines a reusable MEL type
state	Declares source-of-truth data and defaults
computed	Declares values derived from current state
action	Declares a typed transition that callers can request
onceIntent	Makes the action body run once for one submitted action
patch	Writes the next state through the runtime

In Your App

import { createManifesto } from "@manifesto-ai/sdk";
import CounterMel from "./counter.mel";

const app = createManifesto(CounterMel, {}).activate();

await app.action.increment.submit();

const snapshot = app.snapshot();
console.log(snapshot.state.count);      // 1
console.log(snapshot.computed.doubled); // 2

Common Mistake

MEL is not executed like JavaScript. It defines the domain model. TypeScript code activates that model, submits actions, and reads Snapshots.

Next

Read MEL For App Developers for the small syntax set you need before wiring UI, routes, or agents. Then create the runtime in Creating an App and read it in Reading Snapshots. After that, learn what belongs in State and Computed Values. For full syntax, use the MEL Reference.