On Thu, Feb 18, 2016 at 11:05 AM, Edward Kmett
The Functor instance has been around for a decade and is actually fairly heavily used.
It is necessary to support the Applicative for (,) e, which is the anonymous writer monad, like (->) e is the anonymous reader monad, which has also been around for over a decade.
Traversable's sequence gives you canonical distributive law for this functor.
sequence :: (e, f a) -> f (e, a)
The (,) e functor is by far the most common choice of functor for things like lenses to be instantiated with.
forall f. Functor f => (a -> f b) -> s -> f t
becomes (a -> (e, b)) -> s -> (e, t) giving you a secondary result when you want it.
FWIW, as anecdotes have been requested, I use all of these extensively in production, especially that particular usage of sequence.