In this example, we're going to sketch out a blog application using vessel.
First, some preliminaries:
module Tutorial where
import Prelude hiding (id, (.), filter)
import Control.Category
import Control.Lens
import Control.Monad
import Control.Monad.IO.Class
import Control.Monad.Fix
import Data.Aeson.GADT.TH (deriveJSONGADT)
import Data.Align
import Data.Proxy
import Data.Kind (Type)
import Data.Map (Map)
import Data.Map.Monoidal (MonoidalMap(..))
import Data.Semigroup (First(..), Max(..))
import Data.Dependent.Map (DMap)
import Data.Text (Text)
import Reflex
import Reflex.Network
import qualified Data.Map as Map
import qualified Data.Map.Merge.Strict as Map
import Data.Semigroup.Commutative
import Data.Vessel
import Data.Vessel.ViewMorphism
import Data.Vessel.Vessel
import Data.Vessel.Map
import Data.Vessel.Identity
import Data.GADT.Compare.TH
import Data.GADT.Show.TH
import Data.Constraint.Extras.TH
type PostId = Int
type Post = Text
Next we'll define "query" type, which captures the kinds of queries we can have...
data Qsimple g = Qsimple
{ _q_posts :: GrpMap PostId g -- ^ a map from post ID's to refcounts, represents querying for that post
, _q_latestPostId :: GrpMap () g -- ^ morally a "bool"; for if the maxPost Id is being requested.
} deriving (Eq, Ord, Show, Read)
newtype GrpMap k v = GrpMap { unGrpMap :: Map k v } deriving (Eq, Ord, Show, Read)
type role GrpMap nominal nominal
And the corresponding result type. Note that we have the same set of fields occur in both.
data Rsimple = Rsimple
{ _r_posts :: MonoidalMap PostId (First (Maybe Post)) -- ^ posts
, _r_latestPostId :: MonoidalMap () (Max (Maybe PostId)) -- ^ the max post id;
} deriving (Eq, Ord, Show, Read)
Now we end up needing to produce some boilerplate instances for our queries; QueryT (the only "real" instance for MonadQuery) requires that the query type be a Group. It does this for essentially performance reasons. If 100 widgets have queries, and one of them "goes away", then we can either add the remaining 99 queries or subtract the removed query from the total for all 100 we already have. The latter is almost always quicker.
instance (Eq g, Monoid g) => Semigroup (Qsimple g) where Qsimple x y <> Qsimple x' y' = Qsimple (x <> x') (y <> y')
instance (Eq g, Monoid g) => Monoid (Qsimple g) where mempty = Qsimple mempty mempty
instance (Eq g, Group g) => Group (Qsimple g) where negateG (Qsimple x y) = Qsimple (negateG x) (negateG y)
instance (Eq g, Monoid g, Commutative g) => Commutative (Qsimple g)
instance GrpFunctor Qsimple where mapG f (Qsimple x y) = Qsimple (mapG f x) (mapG f y)
class (forall g. (Eq g, Group g) => Group (f g)) => GrpFunctor f where
mapG :: (Eq b, Group b) => (a -> b) -> f a -> f b
instance (Monoid g, Eq g, Ord k) => Semigroup (GrpMap k g) where
GrpMap xs <> GrpMap ys = GrpMap $ Map.merge id id (Map.zipWithMaybeMatched $ const $ liftNonZero (<>)) xs ys
instance (Monoid g, Eq g, Ord k) => Monoid (GrpMap k g) where
mempty = GrpMap Map.empty
instance (Group g, Eq g, Ord k) => Group (GrpMap k g) where
negateG (GrpMap xs) = GrpMap $ fmap negateG xs
GrpMap xs ~~ GrpMap ys = GrpMap $ Map.merge id (Map.mapMissing $ const $ negateG) (Map.zipWithMaybeMatched $ const $ liftNonZero (~~)) xs ys
liftNonZero :: (Monoid a, Eq a) => (a -> a -> a) -> a -> a -> Maybe a
liftNonZero f x y = if (xy /= mempty)
then Just x
else Nothing
where xy = f x y
-- distributive functors can still be groups.
instance GrpFunctor ((->) r) where mapG = fmap
instance GrpFunctor Proxy where mapG = fmap
instance GrpFunctor Identity where mapG = fmap
instance Ord k => GrpFunctor (GrpMap k) where
mapG f (GrpMap xs) = GrpMap $ Map.mapMaybe (\x ->
let fx = f x
in if fx /= mempty
then Just fx
else Nothing) xs
MonadQuery Also requires that QueryResult be a monoid; this reflects the idea that the result can be updated as new data is sent to the frontend; with "updates" being appended to the left. That's the reason for the First and Max values above.
Those are also the reason for the Maybe wrappers in both cases, it's necessary to distinguish the two states of "the data is absent because it doesn't exist in the backend" from "the data is absent because you haven't received it yet".
instance Semigroup Rsimple where Rsimple posts maxId <> Rsimple posts' maxId' = Rsimple (posts <> posts') (maxId <> maxId')
instance Monoid Rsimple where { mempty = Rsimple mempty mempty }
We associate the two types, query and response, with Query; which is also essentially boilerplate code. The single method for Query; crop, should restrict the query result to only that which matches the query. Crop has two essential duties. It's used in query handlers that call runQueryT.
instance Query (Qsimple g) where
type QueryResult (Qsimple g) = Rsimple
crop (Qsimple postsQ maxIdQ) (Rsimple postsR maxIdR) = Rsimple (cropMap postsQ postsR) (cropMap maxIdQ maxIdR)
where cropMap q r = MonoidalMap $ Map.intersection (getMonoidalMap r) (unGrpMap q)
We now can write code that "queries" for posts. Note that the distinction between "not yet loaded" and "doesnt exist at all" is reflected in two different Maybe's. Resist the urge to "join" the two together. That's a sure recipe for annoying glitches which flash "deleted" right before showing the user their data.
Once again we see some amount of boilerplate; we construct the query by building up from the given field; and then need to tear down the query result by examining the corresponding field.
watchPost
:: ( MonadQuery t (Qsimple SelectedCount) m
, QueryResult (Qsimple SelectedCount) ~ Rsimple
, Reflex t
, Monad m
)
=> Dynamic t PostId -> m (Dynamic t (Maybe (Maybe Post)))
watchPost postIds = do
queryResult <- queryDyn $ ffor postIds $ \postId -> mempty { _q_posts = GrpMap (Map.singleton postId 1) }
return $ ffor2 postIds queryResult $ \postId r -> getFirst <$> view (at postId) (_r_posts r)
watchLatestPostId
:: ( MonadQuery t (Qsimple SelectedCount) m
, QueryResult (Qsimple SelectedCount) ~ Rsimple
, Reflex t
, Monad m
)
=> m (Dynamic t (Maybe (Maybe PostId)))
watchLatestPostId = do
queryResult <- queryDyn $ constDyn $ mempty { _q_latestPostId = GrpMap (Map.singleton () 1) }
return $ ffor queryResult $ \r -> getMax <$> view (at ()) (_r_latestPostId r)
displayLatestPost
:: ( MonadHold t m
, MonadFix m
, MonadQuery t (Qsimple SelectedCount) m
, QueryResult (Qsimple SelectedCount) ~ Rsimple
, Reflex t
, PostBuild t m
, Widget t m
)
=> m ()
displayLatestPost = do
mdmId <- maybeDyn =<< watchLatestPostId
dyn_ $ ffor mdmId $ \case
Nothing -> text "Loading ..."
Just dmId -> do
mdId <- maybeDyn dmId
dyn_ $ ffor mdId $ \case
Nothing -> text "No posts found"
Just dId -> displayPost dId
displayPost
:: ( MonadQuery t (Qsimple SelectedCount) m
, QueryResult (Qsimple SelectedCount) ~ Rsimple
, PostBuild t m
, MonadHold t m
, MonadFix m
, Widget t m
)
=> Dynamic t PostId -> m ()
displayPost postId = do
mdmPost <- maybeDyn =<< watchPost postId
dyn_ $ ffor mdmPost $ \case
Nothing -> text "Loading post ..."
Just dmPost -> do
mdPost <- maybeDyn dmPost
dyn_ $ ffor mdPost $ \case
Nothing -> text "Post Not Found"
Just dPost -> dynText dPost
-- To avoid requiring reflex-dom, we stub out a few functions that you'd normally get from reflex-dom-core.
type Widget t m = (NotReady t m, Adjustable t m, PostBuild t m)
dyn_ :: (NotReady t m, Adjustable t m, PostBuild t m) => Dynamic t (m a) -> m ()
dyn_ = void . networkView
text :: Monad m => Text -> m ()
text _ = pure ()
dynText :: Monad m => Dynamic t Text -> m ()
dynText _ = pure ()We can try to improve the situation in essentially all of
these cases above by factoring out the common parts using
something resembling the HKD Pattern; when we need to
associate a group with each query; we can use Const g; and
for the result which demands only the result data for that
key, we can use Identity. A downside is boilerplate
instances, even ones that can normally be derived.
data Qhkd (f :: Type -> Type) = Qhkd
{ _qhkd_posts :: MonoidalMap PostId (f (First (Maybe Post))) -- ^ posts
, _qhkd_latestPostId :: MonoidalMap () (f (Max (Maybe PostId))) -- ^ the max post id;
}
type Qhkd_query g = Qhkd (Const g)
type Qhkd_response = Qhkd Identity
We can instead observe the pattern that "most" of the shape of a record of queries/responses can be decomposed into products of maps. Another way of expressing the same concept is with a DMap. with this approach:
data Qtag (a :: Type) where
Qtag_Posts :: PostId -> Qtag (First (Maybe Post))
Qtag_LatestPostId :: Qtag (Max (Maybe PostId))
type Qtag_query g = DMap Qtag (Const g)
type Qtag_response = DMap Qtag Identity
Vessel takes this idea a bit further; where the above approach uses parameters as values, vessel makes it "recursive"; the GADTs used have "functor" parameters, and most of the applied types are also functor parametric.
data Qvessel (v :: (Type -> Type) -> Type) where
Posts :: Qvessel (MapV PostId (First (Maybe Post)))
LatestPostId :: Qvessel (IdentityV (Max (Maybe PostId)))
Using this sort of construction allows us to eliminate nearly all of the boilerplate; there's a small amount of TH to derive GCompare and all of the remaining instances follow from the view types in vessel:
deriveArgDict ''Qvessel
deriveJSONGADT ''Qvessel
deriveGEq ''Qvessel
deriveGCompare ''Qvessel
deriveGShow ''Qvessel
viewPost :: (MonadQuery t (Vessel Qvessel (Const SelectedCount)) m, Reflex t, Monad m)
=> Dynamic t PostId -> m (Dynamic t (Maybe (Maybe Post)))
viewPost postIds = (fmap.fmap.fmap) (getFirst . runIdentity) $ queryViewMorphism 1 $ ffor postIds $ \pid -> vessel Posts . mapVMorphism pid
viewLatestPostId :: (MonadQuery t (Vessel Qvessel (Const SelectedCount)) m, Reflex t, Monad m)
=> m (Dynamic t (Maybe (Maybe PostId)))
viewLatestPostId = (fmap.fmap.fmap) (getMax . runIdentity) $ queryViewMorphism 1 $ constDyn $ vessel LatestPostId . identityV
Feel free to ignore everything below this line; this is just to force me to get other types "right".
positive :: forall x. (Monoid x, Ord x) => x -> SelectedCount
positive x
| x > mempty = 1
| otherwise = 0
dischargeMonadQuery :: forall v t m a.
( Commutative (v SelectedCount), Group (v SelectedCount), PerformEvent t m, GrpFunctor v, Eq (v SelectedCount)
, Monoid (QueryResult (v SelectedCount)), PostBuild t m, MonadHold t m, MonadFix m, Widget t m
, Query (v SelectedCount)
)
=> (v SelectedCount -> Performable m (QueryResult (v SelectedCount)))
-> (forall m'. (PostBuild t m', MonadHold t m', Widget t m', MonadFix m', MonadQuery t (v SelectedCount) m') => m' a)
-> m a
dischargeMonadQuery getQueryResult widget = mdo
( result
, iVS :: Incremental t (AdditivePatch (v SelectedCount))
) <- runQueryT widget v_t
let
vs_t :: Dynamic t (v SelectedCount) = incrementalToDynamic iVS
dvs :: Event t (v SelectedCount) = attach (current vs_t) (updated vs_t) <&> \(vs_n, vs_n1) -> mapG positive $ mapG positive vs_n ~~ mapG positive vs_n1
pb <- getPostBuild
let vs_0 = tag (current vs_t) pb
v_t <- foldDyn (<>) mempty v_n1
v_n1 :: Event t (QueryResult (v SelectedCount))
<- performEvent $ salign vs_0 dvs <&> \dvs' -> if dvs' /= mempty then return mempty else getQueryResult dvs'
return result
readShowLatestPost
:: ( MonadIO (Performable m)
, PerformEvent t m
, PostBuild t m
, MonadHold t m
, MonadFix m
, Query (Qsimple SelectedCount)
, QueryResult (Qsimple SelectedCount) ~ Rsimple
, Widget t m
)
=> m ()
readShowLatestPost = dischargeMonadQuery promtForIt displayLatestPost
where
promtForIt q = liftIO $ do
print q
readLn