Hal Daume III writes:
| 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.
Ashley Yakeley writes:
| Try this:
|
| class Traversable t a where -- note no fundep
| traverse :: t -> (Maybe a, [t])
:
| instance (Traversable a b, Traversable b c) =>
| Traversable a c where
| traverse = ...
You also have the option of doing something similar in Haskell 98,
without using multi-parameter classes.
class Traversable f where
traverse :: f a -> (Maybe a, [f a])
-- Backward composition of functors
newtype BC f g a = BC {unBC :: f (g a)}
instance (Traversable f, Traversable g) => Traversable (BC f g) where ...
Also see http://www.cse.ogi.edu/~mpj/pubs/springschool.html
Regards,
Tom
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