57.fs

//================================================================================
// 共通
//================================================================================
type EmailAddress =
  { EAValue: string }

type SurfaceAddress =
  { SAValue: string }

type Customer =
  { EmailAddress: EmailAddress
    SurfaceAddress: SurfaceAddress }

type ShoppingCart<'TItem> =
  { Items: 'TItem list }

//================================================================================
// F# way (DU + match(function) expr)
//================================================================================
type Item =
  | FDownloadableItem
  | FSurfaceItem

module FunctionalShipping =
  let ship customer = function
  | FDownloadableItem -> printfn "%sにダウンロードURL付きメールを送信" customer.EmailAddress.EAValue
  | FSurfaceItem -> printfn "%sに配送" customer.SurfaceAddress.SAValue

module FunctionalShoppingCart =
  let empty : ShoppingCart<Item> = { Items = [] }
  let add (item: Item) cart = { cart with Items = item::cart.Items }
  let shipAll customer cart =
    cart.Items
    |> List.iter (FunctionalShipping.ship customer)

//================================================================================
// Objective way(double dispatch(polymorphism))
//================================================================================
type IItem =
  abstract Ship: shipper:ObjectiveShipping * customer:Customer -> unit
and ObjectiveShipping () =
  member __.Ship(item: IItem, emailAddress: EmailAddress) =
    printfn "%sにダウンロードURL付きメールを送信" emailAddress.EAValue
  member __.Ship(item: IItem, surfaceAddress: SurfaceAddress) =
    printfn "%sに配送" surfaceAddress.SAValue

type ODownloadableItem () =
  interface IItem with
    member this.Ship(shipper, customer) =
      shipper.Ship(this, customer.EmailAddress)

type OSurfaceItem () =
  interface IItem with
    member this.Ship(shipper, customer) =
      shipper.Ship(this, customer.SurfaceAddress)

module ObjectiveShoppingCart =
  let empty : ShoppingCart<IItem> = { Items = [] }
  let add (item: IItem) cart = { cart with Items = item::cart.Items }
  let shipAll customer (cart: ShoppingCart<IItem>) =
    let shipper = ObjectiveShipping ()
    cart.Items
    |> List.iter (fun item -> item.Ship(shipper, customer))

//================================================================================
// usage
//================================================================================
let customer =
  { EmailAddress = { EAValue = "admin@example.com" }
    SurfaceAddress = { SAValue = "どこか" } }

// F# way
let fcart =
  FunctionalShoppingCart.empty
  |> FunctionalShoppingCart.add FDownloadableItem
  |> FunctionalShoppingCart.add FSurfaceItem

fcart |> FunctionalShoppingCart.shipAll customer

// Objective way
let ocart =
  ObjectiveShoppingCart.empty
  |> ObjectiveShoppingCart.add (ODownloadableItem())
  |> ObjectiveShoppingCart.add (OSurfaceItem())

ocart |> ObjectiveShoppingCart.shipAll customer