https://medium.com/@riccardoodone/testing-bank-kata-in-purescript-56b1f0a8bb51

Main.purs

data Transaction
	= Deposit Info
	| Withdraw Info

derive instance eqTransaction :: Eq Transaction

instance showTransaction :: Show Transaction where
  show (Deposit i) = show i
  show (Withdraw i) = show i

type Info =
	{ timestamp :: DateTime
 	, amount    :: Int
	}

deposit :: forall m. Monad m => m DateTime -> Int -> StateT (Array Transaction) m Unit
deposit nowDateTime amount = do
  ts <- lift nowDateTime
  let t = Deposit { timestamp: ts, amount: amount }
  modify_ \ts -> ts <> [t]

withdraw :: forall m. Monad m => m DateTime -> Int -> StateT (Array Transaction) m Unit
withdraw nowDateTime amount = do
  ts <- lift nowDateTime
  let t = Withdraw { timestamp: ts, amount: amount }
  modify_ \ts -> ts <> [t]

printStatement :: forall m. Monad m => (String -> m Unit) -> StateT (Array Transaction) m Unit
printStatement logger = do
  s <- gets toStatement
  lift $ logger s

toStatement :: Array Transaction -> String
toStatement =
  fst <<< foldl fnc (Tuple "" 0)
  where
  fnc (Tuple s i) (Deposit d) =
    Tuple (s <> "\n" <> joinWith " " [ show d.timestamp, show d.amount, show $ i + d.amount]) (i + d.amount)
  fnc (Tuple s i) (Withdraw w) =
    Tuple (s <> "\n" <> joinWith " " [ show w.timestamp, "-" <> show w.amount, show $ i - w.amount]) (i - w.amount)

main :: Effect Unit
main = do
  flip evalStateT [] do
    deposit nowDateTime 500
    withdraw nowDateTime 100
    printStatement log