instance (Ord a, Bits a, Bounded a, Integral a, LargeWord a, Bits b, Bounded b, Integral b, LargeWord b) => Bounded (LargeKey a b) where minBound = 0 maxBound = fromIntegral $ (1 + fromIntegral (maxBound::b))* (1 + fromIntegral (maxBound::a)) - 1
Hugs rejects it with +N -98 with
One fix is to bring type variables into the local scope, for example,
instance (Ord a, Bits a, Bounded a, Integral a, LargeWord a, Bits b, Bounded b, Integral b, LargeWord b) => Bounded (LargeKey a b) where minBound = 0 maxBound :: (LargeKey a b) = fromIntegral $ (1 + fromIntegral (maxBound::b))* (1 + fromIntegral (maxBound::a)) - 1
You still need -98 flag for Hugs. Another solution is totally Haskell98: introduce two functions
aoflk:: (LargeKey a b) -> a; aoflk = undefined boflk:: (LargeKey a b) -> b; boflk = undefined
then maxBound can be implemented as
maxBound = result where result = fromIntegral $ (1 + fromIntegral (maxBound `asTypeOf` (boflk result)))* (1 + fromIntegral (maxBound `asTypeOf` (aoflk result))) - 1
The apparent recursion in the above definition is superficial. The definition isn't actually recursive. We merely need the type of the 'result' rather than its value.
Oleg,
Thanks for this. It seems strange that the scope of the universal quantifier is the function definition (in your first solution) rather than the whole of the instance.
Is asTypeOf really Haskell 98?
Dominic.
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