Incorrect type formatting in error messages

I've been trying to write a type signature to something completely analogous to runST . runIdentityT, and failing miserably for a long time.

At some point I had the following:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ImpredicativeTypes #-}

module Test where

import Control.Monad.ST
import Control.Monad.Trans.Identity

runIdST :: IdentityT (forall s. ST s) a -> a
runIdST = runST . runIdentityT

You can see that I was very confused about forall here (probably still am), but what makes it much worse is the error message with which this fails:

src/Test.hs:10:19:
    Couldn't match type `forall s. ST s a' with `forall s. ST s a'
    Expected type: IdentityT (forall s. ST s) a -> forall s. ST s a
      Actual type: IdentityT (forall s. ST s) a -> forall s. ST s a
    In the second argument of `(.)', namely `runIdentityT'
    In the expression: runST . runIdentityT
    In an equation for `runIdST': runIdST = runST . runIdentityT

It says it couldn't match forall s. ST s a with forall s. ST s a. What's up with that?!

This has lead me to a long diversion. Finally, we found the right type for this function (and discovered that the problem is also in the (.), which doesn't have the same special treatment as ($)), and afterwards we realized, that types in the above error message are actually different.

One is (forall s. (ST s)) a, and the other is forall s. ((ST s) a). But they are presented in exactly the same way!

The issue is in the instance Outputable Type, which has too simplistic precedence rules, I guess.

Thanks to @errge for help with debugging this.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Related
Blocking
CC	gergely@risko.hu
Operating system
Architecture