Generalising ($) like Control.Category generalises (.)

Question

I had a thought to generalise ($) like Control.Category generalises (.), and I\'ve done so with the code at the end of this post (also

Answer 1

There are two abstractions used for things like this in Haskell, one usings Arrows and the other Applicatives. Both can be broken down into smaller parts than those used in base.

---

If you go in the Arrow direction and break down the capabilities of Arrows into component pieces, you'd have a separate class for those arrows that are able to lift arbitrary functions into the arrow.

class ArrowArr a where
    arr :: (b -> c) -> a b c

This would be the opposite of ArrowArr, arrows where any arbitrary arrow can be dropped to a function.

class ArrowFun a where
    ($) :: a b c -> (b -> c)

If you just split arr off of Arrow you are left with arrow like categories that can construct and deconstruct tuples.

class Category a => ArrowLike a where
    fst   :: a (b, d) b
    snd   :: a (d, b) b
    (&&&) :: a b c -> a b c' -> a b (c,c')

---

If you go in the Applicative direction this is a Copointed "Applicative without pure" (which goes by the name Apply).

class Copointed p where Source
    copoint :: p a -> a

class Functor f => Apply f where
  (<.>) :: f (a -> b) -> f a -> f b

When you go this way you typically drop the Category for functions and instead have a type constructor C a representing values (including function values) constructed according to a certain set of rules.

Answer 2

$ applies morphisms to values. The concept of a value seems trivial, but actually, general categories need to have no such notion. Morphisms are values (arrow-values... whatever), but objects (types) needn't actually contain any elements.

However, in many categories, there is a special object, the terminal object. In Hask, this is the () type. You'll notice that functions () -> a are basically equivalent to a values themselves. Categories in which this works are called well-pointed. So really, the fundamental thing you need for something like $ to make sense is

class Category c => WellPointed c where
  type Terminal c :: *
  point :: a -> Terminal c `c` a
  unpoint :: Terminal c `c` a -> a

Then you can define the application operator by

($) :: WellPointed c => c a b -> a -> b
f $ p = unpoint $ f . point p

---

The obvious instance for WellPointed is of course Hask itself:

instance WellPointed (->) where
  type Terminal c = ()
--point :: a -> () -> a
  point a () = a
--unpoint :: (() -> a) -> a
  unpoint f = f ()

The other well-known category, Kleisli, is not an instance of WellPointed as I wrote it (it allows point, but not unpoint). But there are plenty of categories which would make for a good WellPointed instance, if they could properly be implemented in Haskell at all. Basically, all the categories of mathematical functions with particular properties (LinK, Grp, {{•}, Top}...). The reason these aren't directly expressible as a Category is that they can't have any Haskell type as an object; newer category libraries like categories or constrained-categories do allow this. For instance, I have implemented this:

instance (MetricScalar s) => WellPointed (Differentiable s) where
  unit = Tagged Origin
  globalElement x = Differentiable $ \Origin -> (x, zeroV, const zeroV)
  const x = Differentiable $ \_ -> (x, zeroV, const zeroV)

As you see, the class interface is actually a bit different from what I wrote above. There isn't one universally accepted way of implementing such stuff in Haskell yet... in constrained-categories, the $ operator actually works more like what Cirdec described.