Ross Paterson wrote:
The Prelude is to obvious place, since that's where both Functor and (->) are introduced.
Yes, but that would break compatibility with Haskell 98.
In that case, we should just create a new module.
OK, one client but no instances.
Fair enough. I like the separation of functionality on principle, but it's difficult to justify without instances. I propose: 1. A new module Data.Functor, exporting Functor and all its instances from the Prelude, as well as "instance Functor ((->) a)". 2. Making Functor a superclass of Applicative, and getting rid of <$>. 3. Making Functor and Foldable superclasses of Traversable, and getting rid of fmapDefault and foldMapDefault. You might also consider: 4. Adding a method to "class Traversable t" (with a default implementation): toList :: t a -> [a] 5. Renaming pure as returnA and <*> as apA. Only it looks like your Arrows stole returnA. Also: If Functor is a superclass of Traversable, is it better to have traverse and mapM as the methods, or sequence and sequenceA? 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 []")? -- Ashley Yakeley, Seattle WA WWED? http://www.cs.utexas.edu/users/EWD/