Thanks .. I will try it out. Meanwhile I could run the following without SqlPersistT but using sqlite-simple directly. runDB :: Member (Embed IO) r => Sem (DB ': r) a -> Sem (Input SQL.Connection ': r) a runDB = reinterpret $ \case QueryAccount ano -> do conn <- input account <- embed $ SQL.queryNamed conn "SELECT * FROM account WHERE account_no = :accountno" [":accountno" := ano] return $ listToMaybe account regards. On Mon, Feb 17, 2020 at 6:42 PM Georgi Lyubenov wrote:

Hi! Sorry for the late reply The "direct" issue is that you're trying to use SqlPersistT as an effect (when using it as an argument to Member in runDB) - Member expects things that are "effects" (produced by polysemy).

If you're not going to be using runDB as an "intermediate" interpreter, but rather running it as a "final" interpreter (to actually run your program) you could instead use something like this:

runDB :: forall b. Members [Embed IO, Input (Pool SqlBackend)] r => SqlPersistT IO b -> Sem r b runDB query = embed . runSqlPool query =<< input

to run your db queries in IO and embed the IO in the resulting effect stack (in this case by also having an input effect from which you can receive your connection pool)

On Fri, Feb 14, 2020 at 11:08 PM Debasish Ghosh wrote:

...

Hello -

I have a typeclass like this ..

class (Monad m) => DB m where query :: Text -> m (Maybe Account) ...

and an instance that uses persistent and sqlite ..

instance (MonadIO m) => DB (SqlPersistT m) where query ano = get (AccountKey ano) ..

Now I want to move this to using an effect system - Polysemy ..

data DB m a where Query :: Text -> DB m (Maybe Account) ...

makeSem ''DB

which generates the functions .. I need some help figuring out the type of the function that interpretes the above .. An initial intuition is this ..

runDB :: Member SqlPersistT r => Sem (DB ': r) a -> Sem r a runDB = interpret $ \case Query ano -> get (AccountKey ano)

which doesn't compile .. errors out with ..

Couldn't match type ‘Control.Monad.Trans.Reader.ReaderT backend0 m0 (Maybe Account)’ with ‘Sem r (Maybe Account)’ Expected type: Sem r x Actual type: Control.Monad.Trans.Reader.ReaderT backend0 m0 (Maybe Account) • In the expression: get (AccountKey accountNo)

I think I am missing something here. The type of interpret is shown as interpret :: (forall x (m :: * -> *). DB m x -> Sem r x) -> Sem (DB : r) a -> Sem r a

But how do I translate the SqlPersistT part ? Any help will be appreciated.

regards. -- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh

Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Debasish Ghosh http://manning.com/ghosh2 http://manning.com/ghosh Twttr: @debasishg Blog: http://debasishg.blogspot.com Code: http://github.com/debasishg