• 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
Emphasis on "rigid". It's not telling you to introduce a new type variable and put that there. It's telling you that the type you need to put there is an existing type variable's type. When you write 'run :: MS c Int a -> (Either String a, Int)' you implicitly mean 'run :: forall c.' which is exactly introducing a new type variable.
• 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) This is the 'c' you bound with the implicit 'forall'. The compiler is asked to verify that 'run' indeed works 'forall c1', so during typechecking of the function body the 'c1' variable is also rigid.
‘c’ is a rigid type variable bound by the type signature for: f1 :: forall (m :: * -> *) c. MonadIO m => c -> m Bool This is the 'c' from the typed hole suggestion up above, still rigid.
A part of the typechecking algorithm is that two rigid type variables cannot be equated. The solution *actually* proposed by GHC in the wildcard suggestion is to use the 'c' variable from 'f1's type for which you need to make it scoped with an explicit 'forall': f1 :: forall c. (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) run = runMS c 0 f2 :: MS c s Bool f2 = pure False f3 :: MS c s [Int] f3 = pure []