[Haskell-cafe] Seeking help with polysemy interpreter ..

[Georgi Lyubenov]

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 <ghosh.debasish at gmail.com>
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
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