cart.fs

type CartItem = string    // placeholder for a more complicated type

type EmptyState = NoItems // don't use empty list! We want to
                          // force clients to handle this as a
                          // separate case. E.g. "you have no
                          // items in your cart"

type ActiveState = { UnpaidItems : CartItem list; }
type PaidForState = { PaidItems : CartItem list; 
                      Payment : decimal}

type Cart = 
    | Empty of EmptyState 
    | Active of ActiveState 
    | PaidFor of PaidForState 

type CartItem = string    // placeholder for a more complicated type

type EmptyState = NoItems // don't use empty list! We want to
                          // force clients to handle this as a
                          // separate case. E.g. "you have no
                          // items in your cart"

type ActiveState = { UnpaidItems : CartItem list; }
type PaidForState = { PaidItems : CartItem list; 
                      Payment : decimal}

type Cart = 
    | Empty of EmptyState 
    | Active of ActiveState 
    | PaidFor of PaidForState 

type EmptyState with
   member this.Add = addToEmptyState 

type ActiveState with
   member this.Add = addToActiveState this 
   member this.Remove = removeFromActiveState this 
   member this.Pay = payForActiveState this 

let addItemToCart cart item =  
   match cart with
   | Empty state -> state.Add item
   | Active state -> state.Add item
   | PaidFor state ->  
       printfn "ERROR: The cart is paid for"
       cart   

let removeItemFromCart cart item =  
   match cart with
   | Empty state -> 
      printfn "ERROR: The cart is empty"
      cart   // return the cart
   | Active state -> 
      state.Remove item
   | PaidFor state ->  
      printfn "ERROR: The cart is paid for"
      cart   // return the cart

let displayCart cart  =  
   match cart with
   | Empty state -> 
      printfn "The cart is empty"   // can't do state.Items
   | Active state -> 
      printfn "The cart contains %A unpaid items"
                                                state.UnpaidItems
   | PaidFor state ->  
      printfn "The cart contains %A paid items. Amount paid: %f"
                                    state.PaidItems state.Payment

type Cart with
   static member NewCart = Cart.Empty NoItems
   member this.Add = addItemToCart this 
   member this.Remove = removeItemFromCart this 
   member this.Display = displayCart this