Hi,
you do realise that "-fallow-incoherent-instances" is enabling a hell. What mostly happens is that the most general instance is chosen. Which explains the error here: Say the confusing instance is chosen (because it is generic) and hence the type checker tries to establish the Confuse constraint, which it can't because the addGeneralFallOut function does not promise it.
I am just finishing up a draft on such class issues having to do with Scrap your boilerplate, which I would be keen to share somewhere later this week.
General conclusion: I still have to see a good reason to use "-fallow-incoherent-instances". It's mostly good to shot yourself in the head.
Ralf
Christian Maeder wrote:
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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