[Haskell-cafe] Applying typed hole proposal leads to compilation failure

[Vlatko Basic]

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 []



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