Oops, nevermind; that was dumb of me. I spoke too quickly. Of course that would produce overlapping instances :)
-- Hal Daume III "Computer science is no more about computers | [EMAIL PROTECTED] than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume On Fri, 18 Jan 2002, Hal Daume III wrote: > In theory, I could write: > > class Traversable d a where > traverse :: d a -> (Maybe a, [d a]) > > instance Traversable Tree a where > traverse (Leaf a) = (Just a, []) > traverse (Branch t1 t2) = (Nothing, [t1,t2]) > > instance Traversable [] a where > traverse [] = (Nothing, []) > traverse (x:xs) = (Just x, [xs]) > > instance (Traversable d (e a), Traversable e a) => Traversable d (e > a) where > traverse = ... > > but then both ghc and hugs complain about overlapping instances, even with > -fallow-overlapping-instances/-fallow-undecidable-instances in ghc and +o > in hugs. > > why would this be? > > - Hal > > -- > Hal Daume III > > "Computer science is no more about computers | [EMAIL PROTECTED] > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > On Fri, 18 Jan 2002, Hal Daume III wrote: > > > I have the following definition: > > > > > class Traversable d where > > > traverse :: d a -> (Maybe a, [d a]) > > > > And the standard binary tree data type: > > > > > data Tree a = Branch (Tree a) (Tree a) > > > | Leaf a > > > > I can define both Tree and [] to be instances of Traversable: > > > > > instance Traversable Tree where > > > traverse (Leaf a) = (Just a, []) > > > traverse (Branch t1 t2) = (Nothing, [t1,t2]) > > > > > instance Traversable [] where > > > traverse [] = (Nothing, []) > > > traverse (x:xs) = (Just x, [xs]) > > > > Now, I want to say that if some data type 'd' is Traversable and another > > data type 'e' is Traversable, then the "combined data type" is > > Traversable. That is, for example, I want to say that a Tree of Lists is > > traversable, or that a List of Trees, or a List of Lists is traversable. > > > > But I can say neither: > > > > > instance Traversable (Tree []) where ... > > > > or: > > > > > instance (Traversable a, Traversable b) => Traversable (a b) where .. > > > > Because of the obvious kind errors. What I suppose I need is some sort of > > lambda expansion over kinds, so I could say: > > > > > instance (Traversable a, Traversable b) => Traversable (\x -> a b x) > > > > or something like that. > > > > Obviously this doesn't exist. > > > > How can I get around this? > > > > - Hal > > > > -- > > Hal Daume III > > > > "Computer science is no more about computers | [EMAIL PROTECTED] > > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > > > > > _______________________________________________ > > Glasgow-haskell-users mailing list > > [EMAIL PROTECTED] > > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users > > > > > _______________________________________________ > Haskell mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell > _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
