gistfile1.hs

#haskell

foldr (.) id [inc, inc, inc] $ 0

data Event 
  = AccountCreated Email
  | AccountLoggedIn
  | Start
   -- .....

foldEvents :: [Event] -> Event
foldEvents = foldl' eval Start

eval :: Event -> Event -> Event
eval Start (AccountCreated email) = undefined
-- patterns ....

class Database a where
  getAll :: Id -> a [Event]

instance Database IO where
  getAll = --- magic happens here

instance Database State where
  getAll = -- get from state

main :: IO ()
main = getDbResult

test :: User -> Test ()
test user = runState getDbResults user

getDbResults user = do
   events <- getAll user 
   return $ foldEvents events


- Set up haskell dev env
Download GHC 7.8.4 with your package manager
mkdir Project
cd Project
cabal init
cabal sandbox init
cabal configure