The Prio Applicative

An interesting Applicative which lets us prioritize computations
Posted in: haskell.


When writing some code recently, I came across a very interesting Applicative Functor. I wanted to write about it for two reasons:

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 .lhs file here).

> {-# LANGUAGE BangPatterns        #-}
> {-# LANGUAGE GADTs               #-}
> {-# LANGUAGE ScopedTypeVariables #-}
> import           Control.Applicative (Applicative (..), (<$>))
> import           Control.Monad       (forM, liftM, liftM2)
> import           Control.Monad.State (State, runState, state)
> import           Unsafe.Coerce       (unsafeCoerce)
> import           Data.List           (sortBy)
> import qualified Data.Map            as M
> import           Data.Ord            (comparing)
> import qualified Data.OrdPSQ         as PSQ
> import           Data.Traversable    (traverse)
> import qualified Data.Vector         as V
> import           GHC.Exts            (Any)

The problem

In our example, we will be modeling a dessert restaurant.

> type Dessert = String

We keep the inventory of our restaurant simply as a list. The important invariant here is that the inventory is always ordered from cheapest to most expensive.

> type Inventory = [Dessert]
> defaultInventory :: Inventory
> defaultInventory =
>     [ "Pancake"
>     , "Apple Pie"
>     , "Apple Pie"
>     , "Tiramisu"
>     ]

Whenever a client wants to order something, they have two options:

In the first case, they will not get served anything if the specific dessert is out of stock. In the second case, they will only miss out on a dessert when our inventory is completely empty.

> data Request
>     = RequestSpecificDessert Dessert
>     | RequestCheapestDessert
>     deriving (Show)

Let’s implement the logic for serving a request. We use State Inventory to keep track of what’s available.

> doRequest :: Request -> State Inventory (Maybe Dessert)

For RequestCheapestDessert, we make use of the fact that our inventory is sorted by price. This means the head of the list is the cheapest dessert, so we serve that and put the tail of the list (xs) back.

We can do that conveniently using the state function, which allows us to modify the state and compute a result at the same time:

state :: (s -> (a, s)) -> State s a

The implementation becomes:

> doRequest RequestCheapestDessert =
>     state $ \inventory -> case inventory of
>         []             -> (Nothing, [])
>         (dessert : xs) -> (Just dessert, xs)

In case the client wants a specific dessert, we use break to take out the requested item from the inventory list.

> doRequest (RequestSpecificDessert requested) =
>     state $ \inventory -> case break (== requested) inventory of
>         (xs, dessert : ys) -> (Just dessert, xs ++ ys)
>         (xs, [])           -> (Nothing, xs)
> test01 = runState (doRequest RequestCheapestDessert) defaultInventory
> test02 = runState (doRequest (RequestSpecificDessert "Apple Pie")) defaultInventory

Let’s check if this works:

*Main> runState (doRequest RequestCheapestDessert) defaultInventory
(Just "Pancake",["Apple Pie","Apple Pie","Tiramisu"])

*Main> runState
    (doRequest (RequestSpecificDessert "Apple Pie")) defaultInventory
(Just "Apple Pie",["Pancake","Apple Pie","Tiramisu"])

Looking good so far!

Because our restaurant wants to make as much money as possible, we choose to first serve the people who order a specific dessert. In order to do that, we have a ‘Priority’ type and each kind of request maps to a priority. Lower numbers means higher priority.

> type Priority = Int
> requestPriority :: Request -> Priority
> requestPriority (RequestSpecificDessert _) = 0
> requestPriority RequestCheapestDessert     = 1

Now let’s see what happens when a bunch of friends visit our restaurant.

> friendsRequests :: [Request]
> friendsRequests =
>     [ RequestCheapestDessert
>     , RequestSpecificDessert "Apple Pie"
>     , RequestCheapestDessert
>     , RequestSpecificDessert "Pancake"
>     , RequestSpecificDessert "Crème brûlée"
>     ]

Easy: we first sort the requests by priority, and then we apply doRequest on every Request. We keep the requests so we know which Dessert corresponds to which Request.

> doRequests :: [Request] -> State Inventory [(Request, Maybe Dessert)]
> doRequests requests =
>     forM (sortBy (comparing requestPriority) requests) $
>         \req -> (,) req <$> doRequest req

Let’s run this for our example to see if it worked and if we got the priorities right:

> test03 = runState (doRequests friendsRequests) defaultInventory
*Main> runState (doRequests friendsRequests) defaultInventory
( [ (RequestSpecificDessert "Apple Pie",    Just "Apple Pie")
  , (RequestSpecificDessert "Pancake",      Just "Pancake")
  , (RequestSpecificDessert "Crème brûlée", Nothing)
  , (RequestCheapestDessert,                Just "Apple Pie")
  , (RequestCheapestDessert,                Just "Tiramisu")
, []

Works great! However, it gets trickier. What if, instead of just a list, we have something with a bit more structure:

> data Family a = Family
>     { familyParent1  :: a
>     , familyParent2  :: a
>     , familyChildren :: V.Vector a
>     } deriving (Show)

And we want to implement:

> doFamilyRequests
>     :: Family Request -> State Inventory (Family (Maybe Dessert))
> doFamilyRequests = error "Implement me"

How do we go about that? Instead of just sorting by priority, we need to tag which request belongs to which parent or child, then sort them, and… it gets messy – especially if the problem becomes more complicated. Imagine, for example, that children get given a bit more priority. It would be cool if we could separate the evaluation order (priority) from our actual logic.

Fortunately, there is an Applicative Functor which solves exactly this problem.

The Prio Applicative

The Prio Applicative has three type parameters:

We use a GADT which mirrors the interface of Applicative, and one additional constructor, which holds a monadic action together with its priority 1.

> data Prio p m a where
>     Pure :: a -> Prio p m a
>     App  :: Prio p m (a -> b) -> Prio p m a -> Prio p m b
>     Prio :: p -> m a -> Prio p m a

For reference, here is the interface of Applicative again:

class Functor f => Applicative f where
    pure  :: a -> f a
    (<*>) :: f (a -> b) -> f a -> f b

We can define a functor instance in terms of Applicative:

> instance Functor (Prio p m) where
>     fmap f = App (Pure f)

And we can use the constructors to implement the Applicative instance:

> instance Applicative (Prio p m) where
>     pure  = Pure
>     (<*>) = App

Now, we probably want to hide the actual constructors from the users and just provide a simple interface. Our interface consists of three functions:

The implementation of prio is straightforward:

> prio :: p -> m a -> Prio p m a
> prio = Prio

A simple implementation of modifyPrio walks through the tree and modifies priorities (Prio nodes) as it encounters them 2.

> modifyPrio :: forall p m a. (p -> p) -> Prio p m a -> Prio p m a
> modifyPrio f = go
>   where
>     go :: forall b. Prio p m b -> Prio p m b
>     go (Pure x)   = Pure x
>     go (App x y)  = App (go x) (go y)
>     go (Prio p x) = Prio (f p) x

runPrio also has a simple implementation: we find the minimal priority, and then evaluate all actions having this priority. When no priorities are left, we can use unsafeEvaluate to evaluate the whole tree 3.

> runPrio :: (Monad m, Ord p) => Prio p m a -> m a
> runPrio os = case findMinimalPriority os of
>     Just p  -> evaluatePriority p os >>= runPrio
>     Nothing -> return $ unsafeEvaluate os

The three auxiliary functions used here findMinimalPriority, evaluatePriority and unsafeEvaluate should be hidden from the user-facing API (except perhaps findMinimalPriority). Let’s look at how these functions are implemented next.

findMinimalPriority simply goes through the Prio value and returns the minimal priority.

> findMinimalPriority
>     :: forall p m a. (Monad m, Ord p)
>     => Prio p m a -> Maybe p
> findMinimalPriority = go Nothing
>   where
>     go :: forall b. Maybe p -> Prio p m b -> Maybe p
>     go !acc        (Pure _)   = acc
>     go !acc        (App x y)  = go (go acc x) y
>     go !Nothing    (Prio p _) = Just p
>     go !(Just !p0) (Prio p _) = Just (min p0 p)

evaluatePriority evaluates all nodes with a priority equal or less than the given priority. We do so by replacing this Prio constructor by a Pure constructor.

> evaluatePriority
>     :: forall p m a. (Monad m, Ord p)
>     => p -> Prio p m a -> m (Prio p m a)
> evaluatePriority p0 = go
>   where
>     go :: forall b. Prio p m b -> m (Prio p m b)
>     go (Pure x)     = return (Pure x)
>     go (App x y)    = liftM2 App (go x) (go y)
>     go (Prio p f)
>         | p <= p0   = liftM Pure f
>         | otherwise = return (Prio p f)

After we have recursively called findMinimalPriority and evaluatePriority until all the Prio nodes are gone, we can call unsafeEvaluate to get our actual value out.

> unsafeEvaluate :: Prio p m a -> a
> unsafeEvaluate (Pure x)   = x
> unsafeEvaluate (App x y)  = (unsafeEvaluate x) (unsafeEvaluate y)
> unsafeEvaluate (Prio _ _) = error
>     "unsafeEvaluate: internal error: some steps still unevaluated"

Usage example

We can now try this out. Remember the type of doRequest:

doRequest :: Request -> State Inventory (Maybe Dessert)

Let’s add a variant which uses the priority of the Request:

> prioRequest :: Request -> Prio Priority (State Inventory) (Maybe Dessert)
> prioRequest req = prio (requestPriority req) (doRequest req)

And for the whole family:

> prioFamilyRequests
>     :: Family Request
>     -> Prio Priority (State Inventory) (Family (Maybe Dessert))
> prioFamilyRequests family = Family
>     <$> prioRequest (familyParent1 family)
>     <*> prioRequest (familyParent2 family)
>     <*> (modifyPrio (\x -> x - 1) $
>             traverse prioRequest (familyChildren family))

Ain’t that clean code. Let’s test it out:

> familyRequest :: Family Request
> familyRequest = Family
>     { familyParent1  = RequestCheapestDessert
>     , familyParent2  = RequestSpecificDessert "Apple Pie"
>     , familyChildren = V.fromList
>           [ RequestCheapestDessert
>           , RequestSpecificDessert "Pancake"
>           , RequestSpecificDessert "Crème brûlée"
>           ]
>     }
> test04 = runState (runPrio $ prioFamilyRequests familyRequest) defaultInventory
*Main> runState (runPrio $ prioFamilyRequests familyRequest)
( Family
    { familyParent1  = Just "Tiramisu"
    , familyParent2  = Just "Apple Pie"
    , familyChildren = fromList
        [ Just "Apple Pie"
        , Just "Pancake"
        , Nothing
, []



Prio is an interesting Applicative. I particularly like the fact that it works for every Monad (although it doesn’t make sense for some Monads such as Reader).

Use cases are rare. I’ve only encountered one and I could also have implemented it in a different way (although this feels a lot cleaner). However, I think a really important point about it is that it really illustrates the difference between Applicative and Monad very well.

Thanks to Alex Sayers, Jared Tobin and Maciej Wos for proofreading and discussions.

Appendix: a faster runPrio

> test05 = runState (fastRunPrio $ prioFamilyRequests familyRequest) defaultInventory

I have been requested to include the code for a faster runPrio, so here it is. As you might expect, it is not as clean as the original one.

The code runs in roughly three steps:

  1. Build a queue which sorts all the elements by priority. In addition to the priority, we have an Int key per Prio node, determined by position.

  2. Evaluate this queue in the arbitrary Monad m. As result we now get a Map which maps this Int key to the value (of type Any).

  3. Run through the original Prio again, and whenever we encounter a Prio node, we use the Int key to lookup and unsafeCoerce the evaluated value from the Map.

> fastRunPrio :: forall p m a. (Monad m, Ord p) => Prio p m a -> m a
> fastRunPrio prio0 = do
>     let (queue, _) = buildQueue 0 prio0 PSQ.empty
>     m <- evaluateQueue queue M.empty
>     let (x, _) = evalPrio m 0 prio0
>     return x
>   where

The three steps are implemented in three auxiliary methods, which you can find here:

>     buildQueue
>         :: forall b.
>            Int
>         -> Prio p m b
>         -> PSQ.OrdPSQ Int p (m Any)
>         -> (PSQ.OrdPSQ Int p (m Any), Int)
>     buildQueue !i (Pure _)    !acc = (acc, i)
>     buildQueue !i (App x y)   !acc =
>         let (acc', i') = buildQueue i x acc in buildQueue i' y acc'
>     buildQueue !i (Prio p mx) !acc =
>         (PSQ.insert i p (liftM unsafeCoerce mx) acc, i + 1)
>     evaluateQueue
>         :: PSQ.OrdPSQ Int p (m Any)
>         -> M.Map Int Any
>         -> m (M.Map Int Any)
>     evaluateQueue q !acc = case PSQ.minView q of
>         Nothing             -> return acc
>         Just (k, _, mx, q') -> do
>             x <- mx
>             evaluateQueue q' (M.insert k x acc)
>     evalPrio
>         :: forall b.
>            M.Map Int Any
>         -> Int
>         -> Prio p m b
>         -> (b, Int)
>     evalPrio m !i (Pure x)    = (x, i)
>     evalPrio m !i (App x y)   =
>         let (x', i')  = evalPrio m i x
>             (y', i'') = evalPrio m i' y
>         in (x' y', i'')
>     evalPrio m !i (Prio p mx) =
>         (unsafeCoerce (m M.! i), i + 1)

  1. It could also be implemented on top of the Free Applicative, but I have decided against that to keep this blogpost as simple as possible.

  2. A faster (but less concise) implementation would be to add a ModifyPrio constructor, and evaluate all of these at once, so we only have to go through the tree once.

  3. This implementation is very slow (quadratic in terms of the number of nodes in the Prio “tree”). I have found a faster way to implement this, but it is again less concise and requires the use of unsafeCoerce, so it is omitted from this blogpost. Update: I have included this method in the Appendix.