Hi,
While working on a toy compiler I realized that Data.Set.Set (Set) is not
an instance of the Functor class. In other words: 'fmap' is not defined
on it. I tried various ways of defining an instance but I failed. The
reason is quite interesting: Set is a type constructor (* -> *) so it
should qualify it for being a Functor. (In a sense it is very similar to
a Map or a list.) However, most Set functions require the elements of the
Set to be an instance of Ord. The problem is that this constraint cannot
be deduced from the instance declaration for Functor:
instance Functor Data.Set.Set where
fmap f s = Data.Set.map f s
GHCi:
Could not deduce (Ord a, Ord b) from the context (Functor
Data.Set.Set)
arising from use of `Data.Set.map' at ...
Probable fix: add (Ord a, Ord b) to the class or instance method
`fmap'
In the definition of `fmap': fmap f s = Data.Set.map f s
In the definition for method `fmap'
In the instance declaration for `Functor Data.Set.Set'
On the other hand, it seems intuitively natural to make Set an instance of
fmap. Any ideas on how to do it?
Thanks and Regards,
Lajos Nagy
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