Try this: {-#LANGUAGE GADTs#-} data Enumerator a b where Enumerator :: a -> a -> Enumerator a a instance Enum a => Foldable (Enumerator a) where foldMap f (Enumerator x y) | fromEnum x > fromEnum y = mempty | otherwise = f x <> foldMap f (Enumerator (succ x) y) Here we're using a GADT to express that our two-parameter Enumerator type in practice always has a == b (at the type level). Which lets us constrain the values inside our new Foldable structure, while still having a type of kind (* -> *) like the the typeclass requires. On Wed, Sep 5, 2018 at 6:56 AM Johannes Waldmann < johannes.waldmann@htwk-leipzig.de> wrote:
Hi David,
Thanks for responding. Let me re-phrase the technical question: in some hypothetical
instance Foldable Enumerator where ...
the methods (e.g., foldMap) would be overconstrained. Is there a way to still write something like it?
It seems not, as shown by these examples:
Data.EnumSet cannot implement Foldable because of Enum k. http://hackage.haskell.org/package/enummapset/docs/Data-EnumSet.html
Data.IntSet cannot implement Foldable because of k ~ Int.
- J. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.