Generic effect and interpose

[mkleczek]

A newbie question. How to handle generic effect like here:

data MyEff a :: Effect where
  MyEff :: a -> MyEff a m ()

type instance DispatchOf (MyEff a) = 'Dynamic

tracedSql :: (MyEff a :> es) => Eff es () -> Eff es ()
tracedSql = interpose $ \env (MyEff a) -> do
    send $ MyEff a

This does not compile with a message: Could not deduce (MyEff a0 :> es) arising from a use of ‘send’
Am I missing something obvious here?

[ocharles]

I believe this is what effectful-plugin is for

[mkleczek]

I see. It should be possible to add some explicit type annotations and have it compiled without the plugin, no?

[arybczak]

Yes, this works:

data MyEff a :: Effect where
  MyEff :: a -> MyEff a m ()
type instance DispatchOf (MyEff a) = Dynamic

tracedSql :: forall a es. (MyEff a :> es) => Eff es () -> Eff es ()
tracedSql = interpose @(MyEff a) $ \env (MyEff a) -> do
    send $ MyEff a

Plugin doesn't really help if there is no type annotation on interpose in this case for some reason, not sure exactly why.

EDIT: It doesn't help because the plugin disambiguates type variables of a particular effect, not effects themselves.

[mkleczek]

Thanks, works brilliantly