Today, we're going to be taking a look at representable functors in Haskell. But first, some incantations:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DataKinds #-}

We recall that a natural transformation between two functors is of the type:

type Nat f g = forall x. f x -> g x

Intuitively, a natural transformation between two functors is a function between functors. for any f x, we have a recipe on how to convert it into g x. An example is the natural transformation from Maybe to List:

-- maybe2list :: Nat Maybe List
maybe2list ::  forall a. Maybe a -> List a
maybe2list Nothing = []
maybe2list (Just a) = [a]

Also recall that the function type a -> b is also called as Hom(a, b) or Hom a b. To stay consistent with category theory terminology, we're going to create a type synonym:

type Hom a b = a -> b

Now, a functor f is saif to be representable if it is the same to Hom a for some a. Let's meditate on this a little bit. So, we have:

1. A functor f
2. An object a, such that f is the same as Hom a
3. A way to convert f into Hom a. That is, we have a natural transformation from f to Hom a. This is the same as asking for Nat f (Hom a), or forall x. f x -> Hom a x, which is the same as forall x. f x -> (a -> x).
4. We also want a way to convert Hom a into f, since f is "the same" as Hom a. This asks for a Nat (Hom a) f, or a forall x. Hom a x -> f x or forall x. (a -> x) -> f x.

Intuitively, this is telling us that there's some object a such that f x is always equal to a -> x for any choice of x. So we get an equivalence beween some "data" f x, and a function a -> x. How do we encode this in haskell? What's an example? Let's see!

Haskell encoding

-- Functor f is representable iff isomorphic to SOME hom functor
-- f: C -> Set -- set to be isomorphic to hom functor
-- ∃d∈C, f ~= Hom(d, -)
-- d is called as the "representing object" of functor f
class Functor f => Representable f where
    type family RepresentingObj f :: *
    -- fwd :: Nat f (Hom d)
    -- fwd :: forall x. f x ->  (Hom d) x
    fwd :: forall x. f x ->  (RepresentingObj f -> x) 
    bwd :: forall x. (RepresentingObj f -> x) -> f x -- memoization

The idea is encoded as shown above, where we say that a functor f is Representable, if (1) it has a RepresentingObj f which represents it, and (2) there are functions to convert f x into (RepresentingObj f -> x).

Prototypical example: Streams

Consider the data type of infinite streams:

-- | always infinite stream
data Stream a = SCons a (Stream a)

for example, the infinite stream of zeros is an inhabitant of this:

zeros :: Stream Int
zeros = SCons 0 zeros

as is the stream of all natural numbers:

nats :: Stream Int
nats = let go n = SCons n (go (n+1)) in go 0

This is a functor, similar to how list is a functor:

instance Functor Stream where
    fmap f (SCons x xs) = SCons (f x) (fmap f xs)

More interestingly, it is a representable functor, since we can think of a Stream a as a function (Integer -> a), as we can index a stream at an arbitrary integer. Similarly, given an (Integer -> a), we can build a Stream a from it by memoizing the function (Integer -> a) into the data structure Stream a:

instance Representable Stream where
    type RepresentingObj Stream = Integer
    -- fwd:: Stream a -> (Integer -> a)
    fwd (SCons x _) 0 = x
    fwd (SCons _ xs) n = fwd xs (n-1)

    -- bwd:: (Integer -> a) -> Stream a
    bwd int2x = go 0 where
        go n = SCons (int2x n) (go (n+1))

Clearly, these are invertible, and thus we witness the equality between Stream a and Integer -> a

Non-example: Lists

Lists are an example of a type that is not a representable functor, because different lists can contain a different numbers of elements. so, for example, we can't use Integer as the representing object (ie, we can't write all lists as functions Natural -> a, because the empty list contains no elements, so it would have to be a function Empty -> a, while the list containing one element has a single element, so it would be the function () -> a, the list containing two elements has two elements, which would be the function Bool -> a, and so on.

Thus, it's impossible for us to find a representing object for a list!

Crazy example: Vectors

Since we saw that lists aren't representable, because we can query "outside their domain", the question naturally becomes, can we build a data structure such that the query remains within the domain? The answer is yes, with much trickery!

First, we'll need an encoding of natural numbers as 0 and a successor function (intuitively, +1), where we represent a number n as adding 1 n times to 0:

data NAT = ZERO | SUCC NAT deriving(Eq)

so, zero will be ZERO, one will be SUCC ZERO, two will be SUCC (SUCC ZERO), and so on. We implement countNAT to convert naturals to integers:

countNAT :: NAT -> Int
countNAT ZERO = 0
countNAT (SUCC n) = countNAT n + 1
instance Show NAT where show n = show (countNAT n)

Next, we create a type FinSet (n :: NAT) :: *, where FinSet n contains EXACTLY n members. Yes, that statement has a lot to unpack:

The type FinSet n has n :: NAT. So we can ask for FinSet ZERO, `FinSet (SUCC ZERO), FinSet (SUCC (SUCC ZERO))` and so on. Notice that in reality, ZERO, SUCC ZERO and so on are values in Haskell. However, we are creating a type FinSet n which depends on values. To enable this, we use the DataKinds extension. Such types which can dependent on values fall under a class of type systems known as dependent types.

Let's first see the definition, and then try and gain some intuition of how this works:

data FinSet (n :: NAT) where
    Intros :: FinSet (SUCC n)
    Lift :: FinSet n -> FinSet (SUCC n)

Why does this work? I claim that FinSet n only has n numbers. Well, let's see. Can I ever crete a member of FinSet ZERO? No, both the constructors Intros and Lift create FinSet (Succ _) for some _, so it's impossible to get my hands on a FinSet ZERO. What about FinSet (SUCC ZERO) [aka FinSet 1]? Yes, there's one way to get such an element:

Intros :: FinSet (SUCC ZERO)

and that's the only way to get a member of FinSet (SUCC ZERO)! What about FinSet (SUCC (SUCC ZERO))? We have two inhabitants:

FinSet (SUCC (SUCC ZERO))

and so on. Here's the Show instance for a FinSet, where we think of Intros as 1, and Lift as incrementing the number.

countFinSet :: FinSet n -> Int
countFinSet Intros = 1
countFinSet (Lift x) = 1 + countFinSet x

instance Show (FinSet n) where
    show x = show (countFinSet x)

We can use the same mechanism to encode Vector n a, a vector with exactly n elements!

data Vector (n :: NAT) (a :: *)  where
    VNil :: Vector ZERO a
    VCons :: a -> Vector n a -> Vector (SUCC n) a

instance Functor (Vector n) where
    fmap _ VNil = VNil
    fmap f (VCons a as) = VCons (f a) (fmap f as)

instance Representable (Vector n) where
    type RepresentingObj (Vector n) = FinSet n

    -- fwd :: forall x. f x ->  (RepresentingObj f -> x) 
    fwd :: forall x. Vector n x -> ((FinSet n) -> x)
    fwd (VCons x xs) Intros = x
    fwd (VCons x xs) (Lift n) = fwd xs n

    bwd :: forall x. ((FinSet n) -> x) -> Vector n x
    bwd finset2x = undefined

Unfortunately, I couldn't quite figure out how to write bwd in Haskell, thus I leave it as an exercise for the reader :)