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Functors and Recursion

Posted on November 19, 2014
Tags: haskell

One of the common pieces of folklore in the functional programming community is how one can cleanly formulate recursive types with category theory. Indeed, using a few simple notions we can build a coherent enough explanation to derive some concrete benefits.

In this post I’ll outline how one thinks of recursive types and then we’ll discuss some of the practical ramifications of such thoughts.


I’m assuming the reader is familiar with some basic notions from category theory. Specifically familiarity with the definitions of categories and functors.

Let’s talk about endofunctors, which are functors whose domain and codomain are the same. spoiler: These are the ones we care about in Haskell. An interesting notion that comes from endofunctors is that of algebras. An algebra in this sense is a pair of an object C, and a map F C → C. Here F is called the “signature” and C is called the carrier.

If you curious about why these funny terms, in abstract algebra we deal with algebras which are comprised of a set of distinguished elements, functions, and axioms called the signature. From there we look at sets (called carriers) which satisfy the specification. We can actually cleverly rearrange the specification for something like a group into an endofunctor! It’s out of scope for this post, but interesting if algebras your thing.

Now we can in fact define a category for F-algebras. in such a category an object is α : F A → A and each arrow is a triplet.

So that f ∘ α = β ∘ F f. In picture form

         F f
F A ———————————————–→ F B
 |                    |
 |                    |
 | α                  | β
 ↓                    ↓
 A —————————————————→ B

commutes. I generally elide the fact that we’re dealing with triplets and instead focus on the arrow, since that’s the interesting bit.

Now that we’ve established F-algebras, we glance at one more thing. There’s one more concept we need, the notion of initial objects. An initial object is an… object, I in a category so that for any object C

 I - - - - - - - - → C

So that f is unique.

Now what we’re interested in investigating is the initial object in the category of F-algebras. That’d mean that

F I ————————————————–→ I
 |                     |
 | F λ                 | λ
 ↓                     ↓
F C —————————————————→ C

Commutes only for a unique λ.

A List is just an Initial Object in the Category of F-Algebras.

What’s the problem?

Now, remembering that we’re actually trying to understand recursive types, how can we fit the two together? We can think of recursive types as solutions to certain equations. In fact, our types are what are called the least fixed point solutions. Let’s say we’re looking at IntList. We can imagine it defined as

    data IntList = Cons Int IntList | Nil

We can in fact, factor out the recursive call in Cons and get

    data IntList a = Cons Int a | Nil
                   deriving Functor

Now we can represent a list of length 3 as something like

    type ThreeList = IntList (IntList (IntList Void))

Which is all well and good, but we really want arbitrary length list. We want a solution to the equation that

X = IntList X

We can view such a type as a set {EmptyList, OneList, TwoList, ThreeList ... }. Now how can we actually go about saying this? Well we need to take a fixed point of the equation! This is easy enough in Haskell since Haskell’s type system is unsound.

    -- Somewhere, somehow, a domain theorist is crying.
    data FixedPoint f = Fix {unfix :: f (FixedPoint f)}

Now we can regain our normal representation of lists with

    type List = FixedPoint IntList

To see how this works

    out :: FixedPoint IntList -> [Int]
    out (Fix f) = case fmap out f of
                    Nil -> []
                    Cons a b -> a : b

    in :: [Int] -> FixedPoint IntList
    in [] = Nil
    in (x : xs) = Fix (Cons x (in xs))

Now this transformation is interesting for one reason in particular, IntList is a functor. Because of this, we can formulate an F-algebra for IntList.

    type ListAlg a = IntList a -> a

Now we consider what the initial object in this category would be. It’d be something I so that we have a function

    cata :: Listalg a -> (I -> a) -- Remember that I -> a is an arrow in F-Alg
    cata :: (List a -> a) -> I -> a
    cata :: (Either () (a, Int) -> a) -> I -> a
    cata :: (() -> a) -> ((a, Int) -> a) -> I -> a
    cata :: a -> (Int -> a -> a) -> I -> a
    cata :: (Int -> a -> a) -> a -> I -> a

Now that looks sort of familiar, what’s the type of foldr again?

    foldr :: (a -> b -> b) -> b -> [a] -> a
    foldr :: (Int -> a -> a) -> a -> [Int] -> a

So the arrow we get from the initiality of I is precisely the same as foldr! This leads us to believe that maybe the initial object for F-algebras in Haskell is just the least fixed point, just as [Int] is the least fixed point for IntList.

To confirm this, let’s generalize a few of our definitions from before

    type Alg f a = f a -> a
    data Fix f = Fix {unfix :: f (Fix f)}

    type Init f = Alg f (Fix f)

    cata :: Functor f => Alg f a -> Fix f -> a
    cata f = f . fmap (cata f) . unfix

Exercise, draw out the reduction tree for cata on lists

Our suspicion is confirmed, the fixed point of an functor is indeed the initial object. Further more, we can easily show that initial objects are unique up to isomorphism (exercise!) so anything that can implement cata is isomorphic to the original, recursive definition we were interested in.

When The Dust Settles

Now that we’ve gone and determined a potentially interesting fact about recursive types, how can we use this knowledge? Well let’s start with a few things, first is that we can define a truly generic fold function now:

    fold :: Functor f => (f a -> a) -> Fix f -> a

This delegates all the messy details of how one actually thinks about handling the “shape” of the container we’re folding across by relegating it to the collapsing function f a -> a.

While this may seem like a small accomplishment, it does mean that we can build off it to create data type generic programs that can be fitted into our existing world.

For example, what about mutual recursion. Fold captures the notion of recurring across one list in a rather slick way, however, recurring over two in lockstep involves a call to zip and other fun and games. How can we capture this with cata?

We’d imagine that the folding functions for such a scenario would have the type

    f (a, b) -> a
    f (a, b) -> b

From here we can build

    muto :: (f (a, b) -> a) -> (f (a, b) -> b) -> Fix f -> (a, b)
    muto f g = cata ((,) <$> f <*> g)

Similarly we can build up oodles of combinators for dealing with folding all built on top of cata!

That unfortunately sounds like a lot of work! We can shamelessly free-load of the hard work of others thanks to hackage though. In particular, the package recursion-schemes has built up a nice little library for dealing with initial algebras. There’s only one big twist between what we’ve laid out and what it does.

One of the bigger stumbling blocks for our library was changing the nice recursive definition of a type into the functorfied version. Really it’s not realistic to write all your types this way. To help simplify the process recursion-schemes provides a type family called Base which takes a type and returns its functorfied version. We can imagine something like

    data instance Base [a] b = Cons a b | Nil

This simplifies the process of actually using all these combinators we’re building. To use recursion-schemes, all you need to is define such an instance and write project :: t -> Base t t. After that it’s all kittens and recursion.

Wrap Up

So dear reader, where are we left? We’ve got a new interesting formulation of recursive types that yields some interesting results and power. There’s one interesting chunk we’ve neglected though: what does unfolding look like?

It turns out there’s a good story for this as well, unfolding is the operation (anamorphism) defined by a terminal object in a category. A terminal object is the precise dual of an initial one. You can notice this all in recursion-schemes which features ana as well as cata.

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