Projection.hs

{-# LANGUAGE TypeFamilies #-}

module Ddd.Projection where

import Prelude hiding (init)
import Data.List (foldl')

class Projection s where

  data Event s :: *

  init  :: s
  apply :: s -> Event s -> s

load :: Projection s => [Event s] -> s
load = foldl' apply init

-- example

newtype State = State Int deriving Show

instance Projection State where

  -- these must come from outside e. g. different module
  data Event State = Increased | Decreased

  init = State 0

  apply (State n) e = State $ case e of
    Increased -> n + 1
    Decreased -> n - 1

--load [Increased, Increased, Decreased, Increased, Increased] -- "State 3"