addMoney.hs

{-# LANGUAGE DataKinds, GADTs, KindSignatures #-}

import Data.Fixed (Centi)

data Currency = USD
              | RUB

data Money :: Currency -> * where
  USDAmount :: Centi -> Money 'USD
  RUBAmount :: Centi -> Money 'RUB


-- (1)
addMoney
  :: Money a
  -> Money a
  -> Money a
addMoney (USDAmount vol1) (USDAmount vol2) =
  USDAmount $ vol1 + vol2
addMoney (RUBAmount vol1) (RUBAmount vol2) =
  RUBAmount $ vol1 + vol2

-- with (1) i get:
-- 
--    Pattern match(es) are non-exhaustive
--    In an equation for ‘addMoney’:
--        Patterns not matched:
--            (USDAmount _) (RUBAmount _)
--            (RUBAmount _) (USDAmount _)


-- vs

-- (2)
addMoney
  :: forall a b. (a ~ Money b)
  => a
  -> a
  -> a
addMoney (USDAmount vol1) (USDAmount vol2) =
  USDAmount $ vol1 + vol2
addMoney (RUBAmount vol1) (RUBAmount vol2) =
  RUBAmount $ vol1 + vol2

-- no warnings with (2)