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fp-ddd chapter 5

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Domain Modeling Made Functional - Chapter 4

Domain Modeling with Types

we’re using a namespace in F# to indicate a DDD bounded context

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namespace​ OrderTaking.Domain
type Undefined = exn
type WidgetCode = WidgetCode of string
type GizmoCode = GizmoCode of string
type ProductCode =
  | Widget of WidgetCode
  | Gizmo of GizmoCode
type UnitQuantikty = UnitQuantity of int
type KilogramQuantity = KilogramQuantity of decimal
type OrderQuantity =
  | Unit of UnitQuantity
  | Kilos of KilogramQuantity
type OrderId = Undefined
type OrderLineId = Undefined
type CustomerId = Undefined
type CustomerInfo = Undefined
type ShippingAddress = Undefined
type ShippingAddress = Undefined
type BillingAddress = Undefined
type Price = Undefined
type BillingAmount = Undefined
type Order = {
  Id: OrderId  // id for entity
  CustomerId: CustomerId  ​// customer reference
  ShippingAddress: ShippingAddress
  BillingAddress: BillingAddress
  OrderLines: OrderLInt list
  AmountToBill: BillingAmount
}
and OrderLine = {
  Id: OrderLineID // id for entity
}

Workflow

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// input
type UnvalidatedOrder = {
  OrderId: string
  CustomerInfo: ...
  ShippingAddress: ...
}
// output
type PlaceOrderEvents = {
  AcknowledgmentSent: ...
  OrderPlaced: ...
  BillableOrderPlaced: ...
}
// Errors
type PlaceOrderError =
  | ValidationError of ValidationError list
  | .. // other errors
and ValidationError = {
  FieldName: string
  ErrorDescription: string
}
// The "place order" process
type PlaceOrder =
  UnvalidatedOrder -> Result<PlaceOrderEvents, PlaceOrderError>