Command

Syntax

command <Name> {
  <fieldName>: <TypeName>
  ...

  validate <expression>
    else "<error message>"
  ...
}

Commands carry intent. Each command declares typed fields and optional validation constraints.

Fields

Fields use name: TypeName syntax. The type references a type declaration within the same context.

Validation

validate constraints define value-level rules checked at command construction time. The grammar accepts validate syntax today — the parser builds AST nodes for each constraint.

The code generator produces Smart Constructors from validate constraints: a private constructor paired with a create() factory returning Result<Command, ValidationError[]>. Invalid commands cannot be instantiated directly.

Multiple validate clauses are allowed. All are checked, and all failures are collected (unlike require guards, which short-circuit).

A command with no validate clauses has a public constructor.

Field Bindings

When emitting events from a decide clause, field bindings pass command data to event fields:

decide(Register, Unregistered) -> [Registered { userId: userId, email: email }]

Shorthand (field name matches command field):

decide(Register, Unregistered) -> [Registered { userId, email, name }]

When the binding name matches a field on the event, the value is inferred from the command’s same-named field.

Example

add-item.ddd

context Ordering {
  type CartId = String
  type ProductId = String
  type Quantity = Integer

  command AddItem {
    cartId: CartId
    productId: ProductId
    quantity: Quantity

    validate quantity > 0
      else "Quantity must be positive"
    validate quantity <= 99
      else "Maximum 99 items per product"
  }

  event ItemAdded {
    cartId: CartId
    productId: ProductId
    quantity: Quantity
  }

  decider Cart {
    commands: AddItem
    events: ItemAdded
    state: Empty | HasItems

    initial: Empty
    terminal: HasItems

    decide(AddItem, Empty) -> [ItemAdded]

    evolve(Empty, ItemAdded) -> HasItems
  }
}