Linear traversables

[b-mehta]

In the past, I'd edited traverse to have type signature

  traverse :: Data.Applicative f => (a ->. f b) -> t a ->. f (t b)

(and similar for sequence) - it now seems this was misguided, since it prevents Const, ((,) a), Either e from being Traversable. (And these last two instances are why Strong (,) () and Strong Either Void are superclasses of Wandering). Instead, the correct signature probably ought to be

  traverse :: Control.Applicative f => (a ->. f b) -> t a ->. f (t b)

This change should not prevent any instance from still working, but might need some definitions to be edited.

[aspiwack]

Ah right, Data.Applicative if the traversable functor only has f and pure data in it. If there is anything else stored in it (like functions, or data of an arbitrary parameter), then you can only traverse it with a Data.Applicative if all that extra data is movable.

This is interesting. Probably switching to Control.Applicative and ropping the Data.Applicative variant altogether is the most reasonable for the moment.