Question

I’ve just found Const in the documentation of Control.Applicative, but I have a hard time working out where this is useful, over just using Monoid directly.

What am I missing?

Answer 1

It’s rather useful when combined with Traversable.

getConst . traverse Const :: (Monoid a, Traversable f) => f a -> a

That’s the general recipe for glomming a bunch of stuff together. It was one of the use cases which convinced me that it was worth separating Applicative from Monad. I needed stuff like generalized elem

elem :: Eq x => x -> Term x -> Bool

to do occur-checking for a Traversable Term parametrized by the representation of free variables. I kept changing the representation of Term and I was fed up modifying a zillion traversal functions, some of which were doing accumulations, rather than effectful mapping. I was glad to find an abstraction which covered both.