When is a generic function not generic?

Question

I'm working on a Haskell server using scotty and persistent. Many handlers need access to the database connection pool, so I've taken to passing the pool around throughout the app, in this sort of fashion:

main = do
    runNoLoggingT $ withSqlitePool ":memory:" 10 $ \pool ->
        liftIO $ scotty 7000 (app pool)

app pool = do
    get "/people" $ do
        people <- liftIO $ runSqlPool getPeople pool
        renderPeople people
    get "/foods" $ do
        food <- liftIO $ runSqlPool getFoods pool
        renderFoods food

where getPeople and getFoods are appropriate persistent database actions that return [Person] and [Food] respectively.

The pattern of calling liftIO and runSqlPool on a pool becomes tiresome after a while - wouldn't it be great if I could refactor them into a single function, like Yesod's runDB, which would just take the query and return the appropriate type. My attempt at writing something like this is:

runDB' :: (MonadIO m) => ConnectionPool -> SqlPersistT IO a -> m a
runDB' pool q = liftIO $ runSqlPool q pool

Now, I can write this:

main = do
    runNoLoggingT $ withSqlitePool ":memory:" 10 $ \pool ->
        liftIO $ scotty 7000 $ app (runDB' pool)

app runDB = do
    get "/people" $ do
        people <- runDB getPeople
        renderPeople people
    get "/foods" $ do
        food <- runDB getFoods
        renderFoods food

Except that GHC complains:

Couldn't match type `Food' with `Person'
Expected type: persistent-2.1.1.4:Database.Persist.Sql.Types.SqlPersistT
                 IO
                 [persistent-2.1.1.4:Database.Persist.Class.PersistEntity.Entity
                    Person]
  Actual type: persistent-2.1.1.4:Database.Persist.Sql.Types.SqlPersistT
                 IO
                 [persistent-2.1.1.4:Database.Persist.Class.PersistEntity.Entity
                    Food]
In the first argument of `runDB', namely `getFoods'

It seems like GHC is saying that in fact the type of runDB becomes specialised somehow. But then how are functions like runSqlPool defined? Its type signature looks similar to mine:

runSqlPool :: MonadBaseControl IO m => SqlPersistT m a -> Pool Connection -> m a

but it can be used with database queries that return many different types, as I was doing originally. I think there's something fundamental I'm misunderstanding about types here, but I have no idea how to find out what it is! Any help would be greatly appreciated.

EDIT:

at Yuras' suggestion, I've added this:

type DBRunner m a = (MonadIO m) => SqlPersistT IO a -> m a
runDB' :: ConnectionPool -> DBRunner m a
app :: forall a. DBRunner ActionM a -> ScottyM ()

which required -XRankNTypes for the typedef. However, the compiler error is still identical.

EDIT:

Victory to the commentors. This allows the code to compile:

app :: (forall a. DBRunner ActionM a) -> ScottyM ()

For which I'm grateful, but still mystified!

The code is currently looking like this and this.

Answer 1

It seems like GHC is saying that in fact the type of runDB becomes specialised somehow.

Your guess is right. Your original type was app :: (MonadIO m) => (SqlPersistT IO a -> m a) -> ScottyM (). This means that your runDB argument of type SqlPersistT IO a -> m a can be used at any one type a. However, the body of app wants to use the runDB argument at two different types (Person and Food) so instead we need to pass an argument that can work for any number of different types in the body. Thus app needs the type

app :: MonadIO m => (forall a. SqlPersistT IO a -> m a) -> ScottyM ()

(I would suggest keeping the MonadIO constraint outside the forall but you can also put it inside.)

EDIT:

What's going on behind the scenes is the following:

(F a -> G a) -> X means forall a. (F a -> G a) -> X, which means /\a -> (F a -> G a) -> X. /\ is the type-level lambda. That is, the caller gets to pass in a single type a and a function of type F a -> G a for that particular choice of a.

(forall a. F a -> G a) -> X means (/\a -> F a -> G a) -> X and the caller has to pass in a function which the callee can specialise to many choices of a.

Answer 2

Lets play the game:

Prelude> let f str = (read str, read str)
Prelude> f "1" :: (Int, Float)
(1,1.0)

Works as expected.

Prelude> let f str = (read1 str, read1 str) where read1 = read
Prelude> f "1" :: (Int, Float)
(1,1.0)

Works too.

Prelude> let f read1 str = (read1 str, read1 str)
Prelude> f read "1" :: (Int, Float)

<interactive>:21:1:
    Couldn't match type ‘Int’ with ‘Float’
    Expected type: (Int, Float)
      Actual type: (Int, Int)
    In the expression: f read "1" :: (Int, Float)
    In an equation for ‘it’: it = f read "1" :: (Int, Float)

But this doesn't. What the difference?

The last f has the next type:

Prelude> :t f
f :: (t1 -> t) -> t1 -> (t, t)

So it doesn't work for clear reason, both elements of the tuple should have the same type.

The fix is like that:

Prelude> :set -XRankNTypes 
Prelude> let f read1 str = (read1 str, read1 str); f :: (Read a1, Read a2) => (forall a . Read a => str -> a) -> str -> (a1, a2)
Prelude> f read "1" :: (Int, Float)
(1,1.0)

Unlikely I can come with good explanation of RankNTypes, so I'd not even try. There is enough resources in web.

Answer 3

To really answer the title question that apparently continues to mystify you: Haskell always chooses the most generic rank-1 type for a function, when you don't supply an explicit signature. So for app in the expression app (runDB' pool), GHC would attempt to have type

app :: DBRunner ActionM a -> ScottyM ()

which is in fact shorthand for

app :: forall a. ( DBRunner ActionM a -> ScottyM () )

This is rank-1 polymorphic, because all type variables are introduced outside of the signature (there is no quantification going on in the signature itself; the argument DBRunner ActionM a is in fact monomorphic since a is fixed at that point). Actually, it is the most generic type possible: it can work with a polymorphic argument like (runDB' pool), but would also be ok with monomorphic arguments.

But it turns out the implementation of app can't offer that generality: it needs a polymorphic action, otherwise it can't feed two different types of a values to that action. Therefore you need to manually request the more specific type

app :: (forall a. DBRunner ActionM a) -> ScottyM ()

which is rank-2, because it has a signature which contains a rank-1 polymorphic argument. GHC can't really know this is the type you want – there's no well defined “most general possible rank-n type” for an expression, since you can always push in extra quantifiers. So you must manually specify the rank-2 type.