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 | hdaume@isi.edu 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 | hdaume@isi.edu 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 | hdaume@isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume
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