https://medium.com/@riccardoodone/bank-kata-in-purescript-5208bd480583

Main.purs

data Transaction
 = Deposit Info
 | Withdraw Info
 
type Info =
 { timestamp :: DateTime
 , amount    :: Int
 }
 
deposit :: Int -> StateT (Array Transaction) Effect Unit
deposit amount = do
  ts <- lift nowDateTime
  let t = Deposit { timestamp: ts, amount: amount }
  modify_ \ts -> ts <> [t]
  
withdraw :: Int -> StateT (Array Transaction) Effect Unit
withdraw amount = do
  ts <- lift nowDateTime
  let t = Withdraw { timestamp: ts, amount: amount }
  modify_ \ts -> ts <> [t]
  
printStatement :: StateT (Array Transaction) Effect Unit
printStatement = do
  s <- gets toStatement
  lift $ log 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 500
    withdraw 100
    printStatement