Hello Cafe, I encountered a strange issue regarding typed hole. If I ask GHC (8.4.3) for a typed hole proposal, it says 'use c', but if I use 'c', it complains that it can't unify 'c' and 'c1'. Why does GHC think those two are different and how to tell to GHC they are the same? Best regards, vlatko *Typed hole proposal:* • Found type wildcard ‘_c’ standing for ‘c’ Where: ‘c’ is a rigid type variable bound by the type signature for: f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool at Test.hs:15:1-32 • In the type signature: run :: MS _c Int a -> (Either String a, Int) *Error when typed hole used:* • Couldn't match type ‘c1’ with ‘c’ ‘c1’ is a rigid type variable bound by the type signature for: run :: forall c1 a. MS c1 Int a -> (Either String a, Int) at Test.hs:21:5-47 ‘c’ is a rigid type variable bound by the type signature for: f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool at Test.hs:15:1-32 Expected type: MS c1 Int a -> (Either String a, Int) Actual type: MS c Int a -> (Either String a, Int) * * *Minimal runnable example:* {-# LANGUAGE ScopedTypeVariables, PartialTypeSignatures #-} module Test where import Prelude import Control.Monad.IO.Class (MonadIO) import Control.Monad.Trans.Except (ExceptT, runExceptT) import Control.Monad.Trans.Reader (Reader, runReader) import Control.Monad.Trans.State (StateT, runStateT) type MS c s = ExceptT String (StateT s (Reader c)) runMS :: c -> s -> MS c s a -> (Either String a, s) runMS c s f = runReader (runStateT (runExceptT f) s) c f1 :: (MonadIO m) => c -> m () f1 c = do let _x1 = run f2 let _x2 = run f3 return () where run :: MS *_c*Int a -> (Either String a, Int) --- failure or success, 'c' or '_' run = runMS c 0 f2 :: MS c s Bool f2 = pure False f3 :: MS c s [Int] f3 = pure []