New type of ($) operator in GHC 8.0 is problematic

[Herbert Valerio Riedel]

On 2016-02-15 at 12:00:23 +0100, Yuras Shumovich wrote:

[...]

>> - It is possible to have unlifted types about even without
>> -XMagicHash. -XMagicHash is simply a lexer extension, nothing more.
>> By convention, we use the # suffix with unlifted things, but there's
>> no requirement here. Having -XMagicHash thus imply a flag about the
>> type system is bizarre.
>
> OK, I always forget about that. But is not it a bug already? Usually we
> don't allow code that uses GHC-specific extensions to compile without a
> language pragma. Why we don't have such pragma for levity
> polymorphism?

There are extensions which are only needed at the definition
site. Take {-# LANGUAGE PolyKinds #-} for instance; this is enabled
inside the Data.Proxy module, which defines the following type

  {-# LANGUAGE PolyKinds #-}
  module Data.Proxy where

  data Proxy t = Proxy

Now when you query via GHCi 7.10, you get the following output

  > import Data.Proxy
  > :i Proxy 
  type role Proxy phantom
  data Proxy (t :: k) = Proxy
    	-- Defined in ‘Data.Proxy’
  instance forall (k :: BOX) (s :: k). Bounded (Proxy s) -- Defined in ‘Data.Proxy’
  instance forall (k :: BOX) (s :: k). Enum (Proxy s) -- Defined in ‘Data.Proxy’
  instance forall (k :: BOX) (s :: k). Eq (Proxy s) -- Defined in ‘Data.Proxy’
  instance Monad Proxy -- Defined in ‘Data.Proxy’
  instance Functor Proxy -- Defined in ‘Data.Proxy’
  instance forall (k :: BOX) (s :: k). Ord (Proxy s) -- Defined in ‘Data.Proxy’
  instance forall (k :: BOX) (s :: k). Read (Proxy s) -- Defined in ‘Data.Proxy’
  instance forall (k :: BOX) (s :: k). Show (Proxy s) -- Defined in ‘Data.Proxy’
  instance Applicative Proxy -- Defined in ‘Data.Proxy’
  instance Foldable Proxy -- Defined in ‘Data.Foldable’
  instance Traversable Proxy -- Defined in ‘Data.Traversable’
  instance forall (k :: BOX) (s :: k). Monoid (Proxy s) -- Defined in ‘Data.Proxy’

even though you never enabled any extensions beyond what Haskell2010 provides.

Do you consider this a bug as well?