I wrote:
If, furthermore, the "confusing instance" is commented out, the source even compiles without extensions.
Correction: -fglasgow-exts is still required for the type GeneralBreakFn, but -fallow-overlapping-instances can be omitted.
{-# OPTIONS -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances -fallow-incoherent-instances #-} module InCoherentInst where
import Control.Exception(throwDyn)
import Data.Typeable(Typeable)
class Confuse a where confuse :: a -> String
instance Confuse a => Typeable a
data GeneralBreakFn a = GeneralBreakFn (forall b . a -> b)
addGeneralFallOut :: Typeable a => GeneralBreakFn a addGeneralFallOut = let breakFn a = throwDyn (GeneralFallOutExcep a) in GeneralBreakFn breakFn
data GeneralFallOutExcep a = GeneralFallOutExcep a deriving (Typeable)