✅ dc-ts

Domain-Centric TypeScript — a functional, event-driven approach to core domain logic in TypeScript.

What is dc-ts?

dc-ts is a lightweight library and architecture style for implementing core domain logic using:

• 🧠 Typed workflows via CoreWf
• 🧪 Type-safe validation using TypeBox
• ♻️ Pure business logic, built around constrain → decide → evolve
• 🔍 Designed for event sourcing and collaborative modeling

Architecture

📦 Core/
 ├─ domain.ts          # your types and workflows
 └─ _implementation/
     ├─ _schema.ts       # generated TypeBox schemas
     ├─ constructors.ts  # smart type-safe constructors
     ├─ constrain.ts     # constraint logic
     ├─ decide.ts        # decision logic (commands → events)
     ├─ evolve.ts        # state transitions (events → state)
     └─ tests/           # BDD-style tests

🧱 CoreWf

At the heart of dc-ts is the CoreWf type:

export type CoreWf<C, iA, E, oA, F> = {
  cmd: C                 // domain command
  inputAg: iA            // aggregate before
  outputAg: oA           // aggregate after
  aggregate: iA | oA
  evt: E                 // emitted event
  fails: F               // failure messages
  validateAggregate: SafeParse<iA | oA>
}

🧪 Example: ToDo Domain

export type CreateToDoWf = CoreWf<
  CMD<'create-todo', { title: string }>,
  AGG<'initial', { title?: string }>,
  EVT<'todo-created', { id: string; title: string }>,
  AGG<'created', { id: string; title: string; status: 'pending' }>,
  'title_required'
>

✅ Constrain

Used to enforce business rules before deciding.

export const constrainToDo: Constrain<CreateToDoWf['cmd'], CreateToDoWf['inputAg'], 'title_required'> =
  (cmd) => (state) => {
    if (!cmd.data.title?.trim()) {
      return fail('title_required')
    }
    return succeed(state)
  }

🧠 Decide

Takes a command and a validated state and returns event(s).

export const decideToDo = (cmd: CreateToDoWf['cmd']) => (state: CreateToDoWf['inputAg']) => {
  const evtRes = newToDoEVT<CreateToDoWf['evt']>()('todo-created')(dtFromMsg(cmd))({
    id: crypto.randomUUID(),
    title: cmd.data.title
  })
  return isFailure(evtRes) ? evtRes : succeed([evtRes.data])
}

🔁 Evolve

Transitions the aggregate to its next state.

export const evolveToDo = (evt: CreateToDoWf['evt']) => (state: CreateToDoWf['inputAg']): CreateToDoWf['outputAg'] => {
  return {
    _tag: 'created',
    data: {
      id: evt.data.id,
      title: evt.data.title,
      status: 'pending'
    }
  }
}

🧪 Validation

All inputs and state transitions are validated using TypeBox:

export const newToDoCMD = <C extends CreateToDoWf['cmd']>() =>
  newCmd<C['type'], C['data']>(safeParseTBox(ToDoCmdSchema))

🧪 BDD Testing

Tests can be generated and follow a Given/When/Then format:

Deno.test("Given a valid command, When constraints run, Then succeed", () => {
  const cmd = { ... }
  const state = { ... }
  const result = constrainToDo(cmd)(state)
  assertEquals(result.outcome, "success")
})

🎯 Design Principles

• 🔄 One source of truth: models power code, tests, and docs
• 🧩 Composable and side-effect free: logic stays pure and testable
• 🛠 Schema-driven development: everything is validated at runtime
• 🧠 Business-first modeling: workflows match EventStorming patterns

🧩 Works Best With

• TypeBox
• dcts CLI: scaffolding, schema generation, test generation, business docs

📘 Need business documentation?

Use the dcts CLI to generate:

• ✅ Business summaries
• 🔁 Decision tables
• 🧪 BDD-style test templates
• 📬 AsyncAPI specs

📝 License

MIT — Giovanni & Contributors