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

[Debasish Ghosh]

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 <godzbanebane at gmail.com>
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 <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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>
>

-- 
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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