-- {-# 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?
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