-- {-# LANGUAGE FunctionalDependencies#-} -- {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} module Register where -- class Register a r | a -> r class Register a where type R a -- instance Register Int Int instance Register Int where type R Int = Int -- instance Register Float Float instance Register Float where type R Float = Float -- instance (Register a1 r1, Register a2 r2) => Register (a1, a2) (r1, r2) instance (Register a, Register b) => Register (a, b) where type R (a, b) = (R a, R b) On 12 Feb 2010, at 00:32, Henning Thielemann wrote:

I have the following class and instance

class Register a r | a -> r where

instance (Register a ra, Register b rb) => Register (a,b) (ra,rb) where

and GHC refuses the instance because of violated Coverage Condition. I have more instances like

instance Register Int8 (Reg Int8) where instance Register Word8 (Reg Word8) where

and for the set of instances I plan, the instance resolution will always terminate. I remember that the term 'undecidable instance' is not fixed and may be relaxed if a more liberal condition can be found. Is there a place, say a Wiki page, where we can collect examples where we think that the current check of GHC is too restrictive? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe