<div dir="ltr">The type of `runPool` is given here: <a href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#v:runPool">https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#v:runPool</a><div><br></div><div>    <a class="gmail-def" id="gmail-v:runPool" style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;background-color:rgb(255,255,0);color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left">runPool</a><span style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>::<span> </span></span><a href="https://www.stackage.org/haddock/lts-12.0/conduit-1.3.0.3/Conduit.html#t:MonadUnliftIO" title="Conduit" style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left">MonadUnliftIO</a><span style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>m => c -><span> </span></span><a href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#t:PersistConfigBackend" title="Database.Persist.Class" style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left">PersistConfigBackend</a><span style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>c m a -><span> </span></span><a href="https://www.stackage.org/haddock/lts-12.0/persistent-2.8.2/Database-Persist-Class.html#t:PersistConfigPool" title="Database.Persist.Class" style="margin:0px;padding:0px;text-decoration:none;font-weight:bold;color:rgb(171,105,84);font-family:monospace;font-size:14px;text-align:left">PersistConfigPool</a><span style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><span> </span>c -> m a</span></div><div><span style="color:rgb(0,0,0);font-family:monospace;font-size:14px;text-align:left;background-color:rgb(240,240,240);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br></span></div><div><span style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The return type of `runPool`, then, is `m a`, for some `m` satisfying `MonadUnliftIO`.</span><br></div><div><span style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br></span></div><div><span style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">The type of `liftIO` is `liftIO :: (MonadIO m) => IO a -> m a`. This means that the first argument to `liftIO` must be an `IO a`.</span></div><div><span style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br></span></div><div><span style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">When we say `liftIO (runPool ...)`, GHC tries to unify the `m a` from `runPool` and the `IO a` from `liftIO`. It is able to do this, as `IO` is an instance of `MonadUnliftIO`. Then, the concrete type of `runPool pgConf f1 pgPool` becomes `IO Bool`.</span></div><div><span style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline"><br></span></div><div><span style="font-size:small;background-color:rgb(255,255,255);text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">GHC now tries to unify the `m` in `f1 :: (MonadIO m, MonadError AppException m) => SqlPersistT m Bool` with `IO`. It tries to satisfy the constraints: `MonadIO` is easily satisfied, but `MonadError AppException m` triggers the problem you mention.</span></div><div><br></div><div>Because `ExceptT` does not have an instance for `MonadUnliftIO`, we cannot use it with the `m` in `SqlPersistT m Bool` directly. We can, instead, use `mapReaderT` to push the `Either` into the return type, like this:</div><div><br></div><div><font face="monospace, monospace">pushException :: SqlPersistT (ExceptT e m) a -> SqlPersistT m (Either e a)</font></div><div><font face="monospace, monospace">pushException = mapReaderT runExceptT</font></div><div><br></div><div><br></div><div>Now, we can write:</div><div><br></div><div><div style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><font face="monospace, monospace">liftIO $ runPool pgConf (pushException f1) pgPool</font></div><div style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">This gives us an `IO (Either AppException Bool)`, which, after lifting, gives us `AppM IO (Either AppException Bool)`. You can then use `either throwError pure` to pull the `AppException` into the `ExceptT` again.</div><div style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">---</div><div style="font-size:small;text-decoration-style:initial;text-decoration-color:initial"><br></div><div style="font-size:small;text-decoration-style:initial;text-decoration-color:initial">I would suggest that you reconsider this approach, however. The `persistent` library uses transactions and exceptions in a way that is *almost always* what you want, but can be surprising, and using `ExceptT` will break this system. Transactions are automatically rolled back on a `MonadCatch`-style exception, but they are not automatically rolled back on an `ExceptT`-style exception. Having a single `AppException` type that represents errors that can occur in database transactions *and* the rest of your application is also going to be a cause for unsafety and errors, as the type cannot possibly be precise enough to provide any safety.</div><br></div></div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div>Matt Parsons</div></div></div></div>
<br><div class="gmail_quote">On Sat, Jul 14, 2018 at 10:22 AM, Vlatko Basic <span dir="ltr"><<a href="mailto:vlatko.basic@gmail.com" target="_blank">vlatko.basic@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
  

    
    
  
  <div style="background-color:rgb(255,255,255);color:rgb(0,0,0)" text="#000000" bgcolor="#FFFFFF">
    <p>Hello,</p>
    <p>I'm trying to use SqlPersistT funcs with MonadError, but am
      failing in writing the runDB correctly. <br>
    </p>
    <p>I tried several things, but always getting:</p>
    <p>    • Couldn't match type ‘IOException’ with ‘AppException’<br>
              arising from a functional dependency between:<br>
                constraint ‘MonadError AppException IO’ arising from a
      use of ‘f1’<br>
                instance ‘MonadError IOException IO’ at <no location
      info><br>
          • In the second argument of ‘runPool’, namely ‘f1’<br>
            In the second argument of ‘($)’, namely ‘runPool pgConf f1
      pgPool’<br>
            In a stmt of a 'do' block: liftIO $ runPool pgConf f1 pgPool</p>
    I understand there already is "instance MonadError IOException IO"
    and fundep says it can be only one for IO.
    <p>How to make it compile?</p>
    <br>
    <p>Best regards,</p>
    <p>vlatko</p>
    <p>    <br>
    </p>
    <p>Here is the minimal reproducible code:<br>
    </p>
    <font size="-1"><tt>module Test where</tt><tt><br>
      </tt><tt><br>
      </tt><tt>import Prelude</tt><tt><br>
      </tt><tt>import Control.Exception.Base</tt><tt><br>
      </tt><tt>import Control.Monad.Except</tt><tt><br>
      </tt><tt>import Control.Monad.Trans.Reader</tt><tt><br>
      </tt><tt>import Database.Persist.Postgresql</tt><tt><br>
      </tt><tt><br>
      </tt><tt>data AppException =</tt><tt><br>
      </tt><tt>    ExcText String</tt><tt><br>
      </tt><tt>  | ExcIO IOException</tt><tt><br>
      </tt><tt>  deriving (Show)</tt><tt><br>
      </tt><tt><br>
      </tt><tt>type AppM m = ExceptT AppException (ReaderT App m)</tt><tt><br>
      </tt><tt>data App    = App { pgConf :: PostgresConf, pgPool ::
        ConnectionPool}</tt><tt><br>
      </tt><tt><br>
      </tt><tt>runDB :: (MonadIO m, MonadError AppException m) =>
        AppM m Bool<br>
      </tt><tt>runDB = do</tt><tt><br>
      </tt><tt>  App{..} <- lift ask</tt><tt><br>
      </tt><tt>  liftIO $ runPool pgConf <b>f1</b> pgPool    -- error</tt><tt><br>
      </tt><tt>  -- liftIO $ runPool pgConf <b>f2</b> pgPool  -- OK</tt><tt><br>
      </tt><tt><br>
      </tt><tt><br>
      </tt><tt>f1 :: (MonadIO m, MonadError AppException m) =>
        SqlPersistT m Bool</tt><tt><br>
      </tt><tt>f1 = throwError $ ExcText "exception"</tt><tt><br>
      </tt><tt><br>
      </tt><tt>f2 :: MonadIO m => SqlPersistT m Bool</tt><tt><br>
      </tt><tt>f2 = return True</tt></font><br>
    <br>
  </div>

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