On Mon, Mar 06, 2006 at 12:36:01PM -0800, Ashley Yakeley wrote:
3. Making Functor and Foldable superclasses of Traversable, and getting rid of fmapDefault and foldMapDefault.
If you have the superclasses, the defaults are more useful, for people who only want to define Traversable.
You might also consider:
4. Adding a method to "class Traversable t" (with a default implementation):
toList :: t a -> [a]
It's in Data.Foldable, but as a function, not a method: toList :: Foldable t => t a -> [a] #ifdef __GLASGOW_HASKELL__ toList t = build (\ c n -> foldr c n t) #else toList = foldr (:) [] #endif
5. Renaming pure as returnA and <*> as apA. Only it looks like your Arrows stole returnA.
I rather like <*>, which comes from Doaitse Swierstra's parsing combinators.
If Functor is a superclass of Traversable, is it better to have traverse and mapM as the methods, or sequence and sequenceA?
sequence(A) would be the categorical way, but it's often more efficient to define traverse/mapM, particularly for non-regular types.
What's your "instance Applicative []"? Does it use repeat and zapp, or is it the "list of successes" (which would be compatible with "instance Monad []")?
It matches the monad instance. There's a newtype ZipList for the other one.