Haskell implementation of Greg Young's CQRS sample: https://github.com/gregoryyoung/m-r Love the sheer elegance of Haskell; no need for all that infrastructure crap

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

There is no neeed for any of that infrastructure crap in C# either, even including DelegateAdjuster. I believe that Greg was just having fun (trolling actually). All that crap, including DelegateAdjuster could be substituted by using dynamic and casting to object.

Stii, on overall, it won't be that terse in C#, fo sure :)

Agreed!

This is more than elegance, it's an electronic poem :) Just a fantastic comparison.