# `DDD.ValueObject`
[🔗](https://github.com/blackuprise/vicander/blob/v0.1.1/lib/ddd/model/value_object.ex#L1)

Behaviour and macros for Domain-Driven Design Value Objects.

Value objects are immutable, identity-less and defined entirely by their
attributes: two value objects with the same attributes are equal. They are
built only through `new/1`, which runs field rules and specification invariants.

## Usage

    defmodule MyApp.Money do
      use DDD.ValueObject

      value_object do
        field :amount,   :decimal, required: true
        field :currency, :string,  required: true

        validate :amount,   &(Decimal.compare(&1, 0) == :gt), message: "must be positive"
        validate :currency, &(String.length(&1) == 3),        message: "must be 3-letter ISO code"
      end
    end

    {:ok, money} = MyApp.Money.new(amount: Decimal.new("10.00"), currency: "USD")

# `equals?`

```elixir
@callback equals?(a :: struct(), b :: struct()) :: boolean()
```

# `new`

```elixir
@callback new(attrs :: map() | keyword()) :: {:ok, struct()} | {:error, DDD.Error.t()}
```

# `value_object`
*macro* 

---

*Consult [api-reference.md](api-reference.md) for complete listing*