[Haskell-cafe] Why is (+++) not a functor?

Hey! =) ArrowChoice's (+++) documentation [1] says that "this is in general not a functor" [1]. What does it mean for (+++) to be or not to be a functor? Cheers, [1] http://hackage.haskell.org/packages/archive/base/4.3.1.0/doc/html/Control-Ar... -- Felipe.