CODEWITHSUNDEEP
postgresqlhaskellhasql
cschneid
cschneid

Reputation: 460

Hasql Session's return value variable getting stuck with specific type?

I am attempting to connect to Postgres once, outside of my Spock application, but I can't seem to use a Hasql session handle with multiple return types.

My main application is fairly straightforward, but fails to compile.

mainApp :: IO Middleware
mainApp = do
  session pgConfig sessConfig $ do
    dbHandle <- sessionUnlifter
    liftIO $ do
      printAHasqlString dbHandle
      printAccountCount dbHandle
      spockT id (appMiddleware >> appRoutes dbHandle)

If I comment out printAHasqlString or printAccountCount, it compiles and works (both work when running without the other).

printAccountCount :: (MonadBase IO m) => (Session Settings s m Int -> IO Int) -> IO ()
printAHasqlString :: (MonadBase IO m) => (Session Settings s m Text -> IO Text) -> IO ()

printAccountCount does a query returning an Int, and printAHasqlString runs a query for a Text. Both just print the result, and returns an IO ().

But when I attempt to run both queries in the same application, the r type variable in the Session data type gets locked down, and fails to compile with the second one.

Error Message:

src/Main.hs:30:25:
  Couldn't match type ‘Text’ with ‘Int’
  Expected type: Session Settings s IO Int -> IO Int
    Actual type: Session Settings s IO Text -> IO Text
  In the first argument of ‘printAccountCount’, namely ‘dbHandle’
  In a stmt of a 'do' block: printAccountCount dbHandle

Updated Error

After some help below - I got to a new error:

src/Main.hs:29:24:
    Couldn't match type ‘r0’ with ‘a’
      because type variable ‘a’ would escape its scope
    This (rigid, skolem) type variable is bound by
      a type expected by the context: Session Settings s IO a -> IO a
      at src/Main.hs:29:7-31
    Expected type: Session Settings s IO a -> IO a
      Actual type: Session Settings s IO r0 -> IO r0
    Relevant bindings include
      dbHandle :: Session Settings s IO r0 -> IO r0
        (bound at src/Main.hs:27:5)
    In the first argument of ‘printAHsqlString’, namely ‘dbHandle’
    In a stmt of a 'do' block: printAHsqlString dbHandle

How do I let that type variable remain flexible between calls?

Full (updated) Code: https://gist.github.com/cschneid/4174addefb254a517f35

Upvotes: 1

Views: 157

Answers (1)

Nikita Volkov
Nikita Volkov

Reputation: 43330

Since version 0.6.0 there no longer is any sessionUnlifter fuss. Pool is directly exposed and you can execute Session on it as many times as you want without any performance penalty.

Upvotes: 2

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