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data Logger a = Logger ([String]->([String], a))

instance Monad Logger where
  return a = Logger (\x->(x, a))
  (Logger a) >>= b = Logger (loggerBind a b)

loggerBind a b prevLog =
  let (newLog, aValue)     = a prevLog
      (Logger newLogger)   = b aValue
      (newLog', newValue') = newLogger newLog
  in (newLog', newValue')

putLog str = Logger (\x->(str:x, ()))

loggedAdd x y = do
  putLog ("Adding " ++ (show x) ++ " and " ++ (show y) ++ "!")
  return (x + y)

loggedSubtract x y = do
  putLog ("Subtracting " ++ (show y) ++ " from " ++ (show x) ++ "!")
  return (x - y)

loggedMul x y = do
  putLog ("Multiplying " ++ (show x) ++ " and " ++ (show y) ++ "!")
  return (x * y)

calculate a b = do
  s <- loggedAdd a b
  sqr <- loggedMul s s
  return sqr

runLogger (Logger a) = a []