Dominic Steinitz wrote:
Did you get the first solution to work? When I tried it with hugs -98 I got
Yes, in the process discovering some interesting behavior of Hugs. Here's the complete code that works with Hugs
module Foo where
class Bits a
instance (Ord a, Bits a, Bounded a, Integral a, Bits b, Bounded b, Integral b) => Bounded (LargeKey a b) where minBound = 0 (maxBound :: (LargeKey a b)) = (fromIntegral::Int->(LargeKey a b)) $ (1 + fromIntegral (maxBound::b))* (1 + fromIntegral (maxBound::a)) - 1 data LargeKey a b = LargeKey a b deriving (Eq, Ord,Show) instance (Ord a, Eq a, Ord b, Show a, Show b) => Num (LargeKey a b) where (+) = undefined fromInteger = undefined
There are two interesting points: first, in order to add a type annotation to the result of a function, we have to place the whole function head in parenthesis, as in (maxBound :: (LargeKey a b)) = ... That does confuse GHC and cause it to give some quite weird error message. So, with parenthesis, it works in Hugs -98 but not in GHC. Without the parenthesis, it works the other way around. The other issue is an unnecessary type annotation on the function fromIntegral. GHC works well without that annotation. Alas, Hugs (November 2003) reports INTERNAL ERROR: findBtyvsInt The second solution seems better: not only it is in Haskell98, it also agrees with both Haskell systems.