[Haskell-cafe] Problem with lifted-async

[Lana Black]

Hello,

I'm writing a thread supervisor that allows implicitly passing some
monadic context (e.g. ReaderT) using MonadBaseControl from
monad-control. The problem is that I don't know how to tackle this
error.

GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( ex.hs, interpreted )

ex.hs:20:20:
    Couldn't match type ‘StM t a0’ with ‘StM t a’
    NB: ‘StM’ is a type function, and may not be injective
    The type variable ‘a0’ is ambiguous
    Expected type: Async (StM t a0)
      Actual type: Async (StM t a)
    Relevant bindings include
      as :: Async (StM t a) (bound at ex.hs:19:30)
      t :: Task t a (bound at ex.hs:19:28)
      td :: TaskDescriptor t (bound at ex.hs:19:10)
      pollTask :: TaskDescriptor t
                  -> TaskDescriptor t -> t (TaskDescriptor t)
        (bound at ex.hs:19:1)
    In the first argument of ‘poll’, namely ‘as’
    In a stmt of a 'do' block: st <- poll as
Failed, modules loaded: none.


This minimal code snippet would be

{-# LANGUAGE ExistentialQuantification  #-}
{-# LANGUAGE FlexibleContexts           #-}

import Control.Concurrent.Async.Lifted (Async,async,asyncBound,poll,cancel)
import Control.Monad.Trans.Control (MonadBaseControl,StM)

data Task m a = Task {
    taskVal   :: m a
}

data TaskDescriptor m = forall a. TaskDescriptor {
    task        :: Task m a,
    asyncThread :: Async (StM m a)
}

runTask :: forall a m. (MonadBaseControl IO m) => Task m a -> m (TaskDescriptor m)
runTask = undefined

pollTask td(TaskDescriptor t as) = do
        st <- poll as
        case st of
            Nothing -> pure td
            Just r -> runTask t

main = undefined



StM is a type from MonadBaseControl typeclass, the definition is

class MonadBase b m => MonadBaseControl b m | m -> b where
    type StM m a :: *
    liftBaseWith :: (RunInBase m b -> b a) -> m a
    restoreM :: StM m a -> m a

type RunInBase m b = forall a. m a -> b (StM m a)

I stripped the comments, the full version is here
https://hackage.haskell.org/package/monad-control-1.0.0.4/docs/src/Control-Monad-Trans-Control.html#MonadBaseControl