Hello,
Apparently I can't do this (in ghc or hugs -- haven't tried others):the problem is, that to make something instance of NormSpace, you must first
class Space a where
distance :: a -> a -> Int
subtract :: a -> a -> a
class (Space a) => NormSpace a where
norm :: a -> Int
distance a b = norm (subtract a b)
That is, I can't make (or redefine) a default for the superclass method
'distance' in a subclass. I agree that the Haskell'98 definition doesn't
claim that this should be allowed, but it seems like something that would be
useful. Are there reasons not to allow this (or that I shouldn't want to do
this at all)?
Apologies if this is well-covered. I couldn't find any mention of it.
have it as instance of Space, i.e. with distance already defined. Then for this
to work, the default definition in NormSpace would have to override what user
defined, which looks strange.
Your problem can be solved in ghc by this (but unfortunately only with
-fallow-overlapping-instances -fallow-undecidable-instances (or some simmilar
flags)):
class HavingSubtract a where -- not sure why generic space should have subtract,
subtract :: a -> a -> a -- but why not
class (HavingSubtract a) => Space a where
distance :: a -> a -> Int
class (HavingSubtract a) => NormSpace a where
norm :: a -> Int
instance (NormSpace a) => Space a where
distance a b = norm (subtract a b)
(and there is several more examples in that I would find something like this
very useful, but AFAIK there is no clean and elegant way how to do it --
you may do something simmilar using newtypes, but that is totally clumsy).
Zdenek
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