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Nathan Howell posts this code:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE RankNTypes #-}
-- Uncomment to compile on GHC 7.8
-- {-# LANGUAGE ImpredicativeTypes #-}
 
module Repro where
 
import Control.Monad.Trans.Cont
import Control.Monad.Trans.State.Lazy
 
newtype AnyContT m a = AnyContT { unAnyContT :: forall r . ContT r m a }
 
class MonadAnyCont b m where
  anyContToM :: (forall r . (a -> b r) -> b r) -> m a
 
instance MonadAnyCont b (AnyContT m) where
  anyContToM _ = error "foo"
 
data DecodeState = DecodeState
 
newtype DecodeAST a = DecodeAST { unDecodeAST :: AnyContT (StateT DecodeState IO) a }
  deriving (MonadAnyCont IO)

Compiling on HEAD produces

[1 of 1] Compiling Repro            ( repro.hs, interpreted )
 
repro.hs:24:13:
    Cannot instantiate unification variable ‛b0’
    with a type involving foralls:
      (forall r. (a1 -> IO r) -> IO r) -> DecodeAST a1
      Perhaps you want ImpredicativeTypes
    In the expression:
        GHC.Prim.coerce
          (anyContToM ::
             (forall (r :: *). (a -> IO r) -> IO r)
             -> AnyContT (StateT DecodeState IO) a) ::
          forall (a :: *).
          (forall (r :: *). (a -> IO r) -> IO r) -> DecodeAST a
    In an equation for ‛anyContToM’:
        anyContToM
          = GHC.Prim.coerce
              (anyContToM ::
                 (forall (r :: *). (a -> IO r) -> IO r)
                 -> AnyContT (StateT DecodeState IO) a) ::
              forall (a :: *).
              (forall (r :: *). (a -> IO r) -> IO r) -> DecodeAST a
Failed, modules loaded: none.

Two questions:

1. Should we require users to specify !ImpredicativeTypes here? Or, should the !GeneralizedNewtypeDeriving mechanism (which sometimes is impredicative) just assume the extension?
2. Can we improve the error message?

I'd like to note that the original example really does require impredicativity -- the question is whether and how to bother the user with this technicality.

Small program note: I've done a lot of stuff with !GeneralizedNewtypeDeriving lately but am on holiday until Jan. 6, so don't expect responses!

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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