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MEL ​

Purpose: Overview of MEL (Manifesto Expression Language) Audience: Anyone writing or reviewing Manifesto domains Reading time: 5 minutes

What is MEL? ​

MEL (Manifesto Expression Language) is the declarative, typed file format for a Manifesto domain. You write domain state, computed values, actions, and effect declarations in .mel; your app imports that domain through the compiler and activates it with the SDK runtime.

MEL is designed for:

  • human-authored app domains
  • AI-assisted domain drafts that humans can review
  • build-time validation before runtime code runs

Where MEL Fits ​

MEL source -> compiler integration -> createManifesto() -> app.action.* -> snapshot()

MEL is a source format. It does not execute like JavaScript. App code activates the compiled domain, submits actions, and reads Snapshots.

If you are writing your first app, read MEL For App Developers before using the complete reference pages.

This docs section prefers the current sugar-first MEL surface in examples: count + 1, items[id], a ? b : c, and { ...base, status: "done" }. Equivalent function-form source is still documented, but it is folded into the reference instead of leading every example.

What MEL is NOT ​

  • Not a general-purpose language
  • Not a workflow engine
  • Not an effect executor
  • Not a runtime

Quick Example ​

mel
domain Counter {
  state {
    count: number = 0
  }

  computed doubled = count * 2

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

Tip: Use onceIntent for per-intent idempotency without schema guard fields. Use once() only when you need an explicit guard field in domain state.

Why MEL? ​

BenefitDescription
Human-readableClean syntax that's easy to read and review
LLM-friendlyStructured enough for AI to generate and validate
Build-time validationCatch errors before runtime with compiler checks
Deterministic outputSame source always produces the same compiled domain

Under the hood, MEL compiles to a DomainSchema, which the runtime computes deterministically. App code normally stays on .mel imports, createManifesto(), app.action.*, and snapshot().

CLI: check and compile ​

bash
pnpm add -D @manifesto-ai/compiler
pnpm exec mel check path/to/domain.mel
pnpm exec mel compile path/to/domain.mel --stdout

App Path ​

Reference ​

Maintainers And Tool Authors ​