On Sat, Jan 19, 2019 at 04:20:23PM -0500, Viktor Dukhovni wrote:

in my complete program, the call to 'mkEnv' fails to compile, when called via:

mkLockEnv :: IO Env mkLockEnv = do lock <- newMVar () let locker :: Locker locker = withMVar lock . const ... (mkEnv locker)

But after inlining the "locker" binding, the code compiles:

mkLockEnv :: IO Env mkLockEnv = do lock <- newMVar () ... (mkEnv (withMVar lock . const))

Given "let-bound polymorphism":

https://kseo.github.io/posts/2016-12-27-higher-rank-polymorphism.html

I would not have expected the change to make a difference...

Well, it turns out that some of the difference between the simplified and complete program is that my "Reader Env IO" monad gets some additional constraints via: http://hackage.haskell.org/package/http-conduit-2.3.4/docs/Network-HTTP-Clie... withResponse :: ( MonadUnliftIO m, MonadIO n, MonadReader env m , HasHttpManager env) => Request -> (Response (ConduitM i ByteString n ()) -> m a) -> m a Removing the call to 'withResponse' allows a simplified program compile without inlining the let-bind. Inspired by that, I cobbled together the below, which fails to compile unless one either uncomments the explicit type declaration for the let-bind, or else inlines the value: {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} module Main (main) where import Control.Monad.IO.Unlift (MonadUnliftIO, withRunInIO) import Control.Concurrent.MVar (newMVar, withMVar) import Control.Monad.IO.Class (liftIO) import Control.Monad.Reader (MonadReader, ReaderT, runReaderT, asks) data Env = Env { envLocker :: !Locker, envString :: String } type Locker = forall a. IO a -> IO a runLocked :: (env ~ Env, MonadReader env m, MonadUnliftIO m) => forall a. m a -> m a runLocked action = asks envLocker >>= \locker -> withRunInIO $ \run -> locker $ run action -- XXX: To compile, uncomment let-bind type, or else inline! mkEnv :: IO Env mkEnv = newMVar () >>= \lock -> let -- locker :: Locker locker = withMVar lock . const in go locker "Hello World!" where go :: Locker -> String -> IO Env go envLocker envString = Env{..} main :: IO () main = mkEnv >>= runReaderT (runLocked $ asks envString >>= liftIO . putStrLn) And yet, adding a type declaration for the let-bind still is not enough for the full program, only inlining "withMVar lock . const" makes GHC happy. Don't yet know why... • Couldn't match type ‘a’ with ‘a0’ ‘a’ is a rigid type variable bound by a type expected by the context: forall a. IO a -> IO a at Jname.hs:135:32-56 Expected type: IO a -> IO a Actual type: IO a0 -> IO a0 • In the second argument of ‘optsParser’, namely ‘locker’ In the second argument of ‘(<*>)’, namely ‘optsParser manager locker’ In the first argument of ‘O.info’, namely ‘(O.helper <*> optsParser manager locker)’ • Relevant bindings include locker :: IO a0 -> IO a0 (bound at Jname.hs:132:9) | 135 | $ O.info (O.helper <*> optsParser manager locker) | ^^^^^^ -- Viktor.