Let's say you have a simple pure function that applies a discount of 30% if the total price of a list of Items is over 30.00 (let's not delve into the fact that I'm using a Float to indicate the price):

promoGet30PercentOff :: [Item] -> Float
promoGet30PercentOff xs
  | total > 30.0 = total * 0.7
  | otherwise    = total
  where total = sum $ map itemPrice xs

and Item and itemPrice are defined as follows:

data Item = Apple | Banana deriving Show

itemPrice :: Item -> Float
itemPrice x =
  case x of
    Apple -> 5.9
    Banana -> 3.0

I'd now like to test promoGet30PercentOff at the boundary. As I come from Python, my natural reaction would be to mock Item and create some that are priced at 30.0, 30.1 and 0.0, for instance.

I believe, this is not possible in Haskell (but correct me if I'm wrong). How would you go about it then?


1 Answer 1


You can untangle your promo function from the Item data type

promoGet30PercentOffWith :: (a -> Float) -> [a] -> Float
promoGet30PercentOffWith price xs
  | total > 30.0 = total * 0.7
  | otherwise    = total
  where total = sum $ map price xs

promoGet30PercentOff :: [Item] -> Float
promoGet30PercentOff = promoGet30PercentOffWith itemPrice

Because promoGet30PercentOffWith is polymorphic, there is a free theorem that all functions of its type satisfy:

promoGet30PercentOffWith f xs = promoGet30PercentOffWith id (map f xs)

This means that the general behavior of that function is entirely determined by its specialized behavior when the first argument is id---so the list argument has type [Float]. Thus, mocking items with artificial prices is as simple as giving the list of prices to the function.

shouldPromo :: [Float] -> IO ()
shouldPromo xs = promoGet30PercentOffWith id xs === sum xs * 0.7

shouldNotPromo :: [Float] -> IO ()
shouldNotPromo xs = promoGet30PercentOffWith id xs === sum xs

(===) :: Eq a => a -> a -> IO ()
x === y = if x == y then return () else error "assertion failed"

test :: IO ()
test = do
  shouldPromo [30.1]
  shouldNotPromo [30.0]
  shouldNotPromo [0]

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