data CurrencyType = GBP | USD | EUR
  deriving (Show)

data Currency = Currency CurrencyType Int
  deriving (Show)

gt :: MonadError String m => Currency -> Currency -> m Bool
gt (Currency k1 v1) (Currency k2 v2)
  | k1 == k2 = pure $ v1 > v2
  | otherwise = throwError "Cannot compute 'gt' on different currency kinds"

add :: MonadError String m => Currency -> Currency -> m Currency
add (Currency k1 v1) (Currency k2 v2)
  | k1 == k2 = pure $ Currency k1 (v1 + v2)
  | otherwise = throwError "Cannot compute 'add' on different currency kinds"

subtract :: MonadError String m => Currency -> Currency -> m Currency
subtract (Currency k1 v1) (Currency k2 v2)
  | k1 == k2 = pure $ Currency k1 (v1 - v2)
  | otherwise = throwError "Cannot compute 'subtract' on different currency kinds"