-fallow-incoherent-instances

[Christian Maeder]

The attached module does not compile and yields the following error:
InCoherentInst.hs:17:
    Could not deduce (Confuse a) from the context (Typeable a)
      arising from use of `breakFn' at InCoherentInst.hs:17
    Probable fix:
        Add (Confuse a) to the type signature(s) for `addGeneralFallOut'
    In the first argument of `GeneralBreakFn', namely `breakFn'
    In the definition of `addGeneralFallOut':
        addGeneralFallOut = let
                              breakFn a = throwDyn (GeneralFallOutExcep a)
                            in GeneralBreakFn breakFn
The same source compiles ok without -fallow-incoherent-instances (or 
with -fno-allow-incoherent-instances).

If, furthermore, the "confusing instance" is commented out, the source 
even compiles without extensions.

I don't know if this is a bug, possibly related to the import of 
Typeable stuff. I don't need a fix. I only want to point out that 
globally switching on the option -fallow-incoherent-instances is likely 
to break existing code, currently (ghc 6.2.2).

Cheers Christian
{-# 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)
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