Hi mniip,

Let me first apologise for my very late response. I went for a visit to the analog world, and stayed much longer than planned. :-)

I have ScopedTypeVariables enabled as a default extension in .cabal file, but have never encountered such an error, to have to manually specify forall just for making scoped types to work.

I'm using local signatures quite often, but still not quite clear as to how/where the original code differs, for example, from this one (which compiles fine):

mkTransUnitValTag :: (HasGlobals s) => InNode -> MS c s TransUnitValT
mkTransUnitValTag e@(Element "tuv" as (cleanBlank -> cs) _) = do
  TransUnitValT <$> attrGlobDef e glbDataType "datatype"   as -- tuvDataType

                 ...
                <*> parseTag    e "seg" mkSegTag           cs -- tuvSeg
  where
    mkSegTag :: InNode -> MS c s Content
    mkSegTag (Element "seg" _as ss _) = checkContent =<< mapM mkContentTag ss

Is the main diff that 'run' is having monad stack as input and is running it, while 'mkSegTag' is run in it (so forall does not have to be specified manually)?

    f1 :: forall m c. (MonadIO m) => c -> m ()  -- original code

       where run :: MS c Int a -> (Either String a, Int)

Thanks for pointing me to read the whole error/warning. Everything is actually written there, but seems I have developed some kind of forall blindness. :-(

• 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 []
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