ImpredicativeTypes vs. LiberalTypeSynonyms

[MartijnVanSteenbergen]

Consider the following module:

{-# LANGUAGE Rank2Types #-}

module RunMud where

import Control.Monad.State

type Mud      = StateT MudState IO
data MudState = MudState { mRunMud :: RunMud }
type RunMud   = forall a. Mud a -> IO a

getRunMud :: Mud RunMud
getRunMud = do
  s <- get
  return (mRunMud s)

GHC suggests:

Illegal polymorphic or qualified type: RunMud
Perhaps you intended to use -XImpredicativeTypes

When I add a language pragma for ImpredicativeTypes?, GHC stills comes up with the same error message.

Two questions:

1) Can GHC please not suggest enabling extensions that are already enabled? It's very confusing.

2) Can GHC in this specific case suggest LiberalTypeSynonyms?? That's the third extension that needed to be on for this example to compile.

[simonpj]

Happily, the HEAD has this fixed already

$ $vpj -c T3145.hs

T3145.hs:11:0:
    Illegal polymorphic or qualified type: RunMud
    Perhaps you intended to use -XLiberalTypeSynonyms
    In the type signature for `getRunMud':
      getRunMud :: Mud RunMud

[MartijnVanSteenbergen]

That's awesome; thank you!