InventoryItems.hs

module InventoryItems(Command(..), Event(..), handle) where

import Data.Maybe(isJust)

type Id = String
type Name = String
type Amount = Int

data Command = CreateInventoryItem	Id
	     | RenameInventoryItem	Id Name
	     | RemoveItemsFromInventory	Id Amount
	     | AddItemsToInventory	Id Amount
	     | DeactivateInventoryItem	Id
	     deriving (Show)

data Event  = InventoryItemCreated 	Id
	    | InventoryItemRenamed 	Id Name
	    | ItemsRemovedFromInventory Id Amount
	    | ItemsCheckedInToInventory Id Amount
	    | InventoryItemDeactivated	Id
	    deriving (Show, Read)

data Item = Item { itemId        :: Id
                 , itemActivated :: Bool
		 }

handle :: [Event] -> Command -> [Event]
handle events command = 
	handle' item command 
	where item = apply events

handle' maybeItem (CreateInventoryItem id)
	| itemExists 	= error "item already created"
	| otherwise	= [InventoryItemCreated id]
	where itemExists = isJust maybeItem

handle' Nothing _command =  error "please create the item first"

handle' (Just item) (DeactivateInventoryItem id)
	| deactivated	= error "already deactivated"
	| otherwise 	= [InventoryItemDeactivated id]
	where deactivated = not (itemActivated item)

handle' (Just Item {itemActivated = False}) _command = error "item is deactivated"

handle' (Just item) (RenameInventoryItem id name) 
	| invalidName	= error "newName"
	| otherwise	= [InventoryItemRenamed id name]
	where invalidName = name == []

handle' (Just item) (RemoveItemsFromInventory id amount)
	| invalidAmount	= error "can't remove negative amount from inventory"
	| otherwise 	= [ItemsRemovedFromInventory id amount]
	where invalidAmount = amount <= 0

handle' (Just item) (AddItemsToInventory id amount)
	| invalidAmount	= error "must have an amount > 0 to checkin to inventory"
	| otherwise 	= [ItemsCheckedInToInventory id amount]
	where invalidAmount = amount <= 0

apply events = foldl apply' Nothing events

apply' Nothing (InventoryItemCreated id)		= Just (Item id True)
apply' (Just item) (InventoryItemDeactivated id)	= Just (item {itemActivated = False})
apply' (Just item) _ 					= Just item