Example of Haskell code I see under tested a lot

Order.hs

module Order where

-- DBM is some Monad Transformer stack that allows us to talk to our DB (also a form of DI!)

placeOrder :: User -> OrderDetails -> DBM ()
placeOrder user details = do
  -- Some users will have discounts for 
  (discount :: Maybe Discount) <- findRelevantDiscount user details

  -- Create the specific order for this user with any discount
  insertOrder $ mkOrder user details discount

  -- Ensure any discount used won't be used again
  forM_ discount $ redeemDiscount 
  

-- Consider the properties we might want to test, e.g.

-- If there's a discount, and the order is place, the discount must be redeemed
-- if there's a discount, and the order errors, the discount must not be redeemed

-- These are things we can property test, except doing that against a
-- DB connection, makes this a hard thing to do. This is where MTL style type classes shine. 

-- A naive function parameters version would look like

placeOrderFunction :: (User -> OrderDetails -> DBM (Maybe Discount)) -- findRelevantDiscount
                   -> (OrderRecord -> DBM ()) -- insertOrder
                   -> (Discount -> DBM ()) -- redeemDiscount
                   -> User -> OrderDetails -> DBM ()
            
-- An MTL style looks like this, and can be tested using monad-mock https://hackage.haskell.org/package/monad-mock
placeOrderMtl :: (MonadDB m) => User -> OrderDetails -> m ()