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@NathanHowell NathanHowell/error.txt
Last active Jan 1, 2016

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GNTD regressions in GHC HEAD. This typechecks on 7.6.3 but not on 7.7.20131217 unless ImpredicativeTypes are enabled.
[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.
{-# 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)
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