On Ad-hoc Datatypes
Published on May 11, 2016 under the tag haskell
In Haskell, it is extremely easy for the programmer to add a quick datatype. It does not have to take more than a few lines. This is useful to add auxiliary, ad-hoc datatypes.
I don’t think this is used enough. Most libraries and code I see use “heavier” datatypes: canonical and very well-thought-out datatypes, followed by dozens of instances and related functions. These are of course great to work with, but it doesn’t have to be a restriction: adding quick datatypes – without all these instances and auxiliary functions – often makes code easier to read.
The key idea is that, in order to make code as simple as possible, you want to represent your data as simply as possible. However, the definition of “simple” is not the same in every context. Sometimes, looking at your data from another perspective makes specific tasks easier. In those cases, introducing “quick-and-dirty” datatypes often makes code cleaner.
This blogpost is written in literate Haskell so you should be able to just load
it up in GHCi and play around with it. You can find the raw
import Control.Monad (forM_)
Let’s look at a quick example. Here, we have a definition of a shopping cart in a fruit store.
data Fruit = Banana | Apple | Orange deriving (Show, Eq) type Cart = [(Fruit, Int)]
And we have a few functions to inspect it.
cartIsHomogeneous :: Cart -> Bool = True cartIsHomogeneous  : xs) = all ((== fruit) . fst) xscartIsHomogeneous ((fruit, _)
cartTotalItems :: Cart -> Int = sum . map sndcartTotalItems
This is very much like code you typically see in Haskell codebases (of course, with more complex datatypes than this simple example).
The last function we want to add is printing a cart. The exact way we format it depends on what is in the cart. There are four possible scenarios.
- The cart is empty.
- There is a single item in the customers cart and we have some sort of simplified checkout.
- The customer buys three or more of the same fruit (and nothing else). In that case we give out a bonus.
- None of the above.
This is clearly a good candidate for Haskell’s
case statement and guards.
Let’s try that.
printCart :: Cart -> IO () = case cart of printCart cart -> putStrLn $ "Your cart is empty"  1)] -> putStrLn $ "You are buying one " ++ show fruit [(fruit, | cartIsHomogeneous cart && cartTotalItems cart >= 3 -> do _ putStrLn $ show (cartTotalItems cart) ++ " " ++ show (fst $ head cart) ++ "s" ++ " for you!" putStrLn "BONUS GET!" | otherwise -> do putStrLn $ "Your cart: " $ \(fruit, num) -> forM_ cart putStrLn $ "- " ++ show num ++ " " ++ show fruit
This is not very nice. The business logic is interspersed with the printing
code. We could clean it up by adding additional predicates such as
cartIsBonus, but having too many of these predicates leads to a certain kind
of Boolean Blindness.
Instead, it seems much nicer to introduce a temporary type:
data CartView = EmptyCart | SingleCart Fruit | BonusCart Fruit Int | GeneralCart Cart
This allows us to decompose our
printCart into two clean parts: classifying
the cart, and printing it.
cartView :: Cart -> CartView = EmptyCart cartView  1)] = SingleCart fruit cartView [(fruit, cartView cart| cartIsHomogeneous cart && cartTotalItems cart >= 3 = BonusCart (fst $ head cart) (cartTotalItems cart) | otherwise = GeneralCart cart
Note that we now have a single location where we classify the cart. This is
useful if we need this information in multiple places. If we chose to solve the
problem by adding additional predicates such has
cartIsBonus instead, we would
still have to watch out that we check these predicates in the same order
everywhere. Furthermore, if we need to add a case, we can simply add a
constructor to this datatype, and the compiler will tell us where we need to
update our code 1.
printCart becomes very simple now:
printCart2 :: Cart -> IO () = case cartView cart of printCart2 cart EmptyCart -> putStrLn $ "Your cart is empty" SingleCart fruit -> putStrLn $ "You are buying one " ++ show fruit BonusCart fruit n -> do putStrLn $ show n ++ " " ++ show fruit ++ "s for you!" putStrLn "BONUS GET!" GeneralCart items -> do putStrLn $ "Your cart: " $ \(fruit, num) -> forM_ items putStrLn $ "- " ++ show num ++ " " ++ show fruit
Of course, it goes without saying that ad-hoc datatypes that are only used locally should not be exported from the module – otherwise you end up with a mess again.
If you are compiling with
-Wall, which is what you really, really should be doing.↩︎