Hylomorphisms and treesort

Consider the following data structure, representing a binary search tree:

data  BST a = 
    Tree (BST a) a (BST a)
  | Empty deriving (Eq)

As it turns out, this data structure provides a nice way to introduce the concepts of different types of morphisms used all over the place in Haskell - the fold, or “catamorphism”, the unfold, or “anamorphism”, and compositions of the two, the “hylomorphism” and “metamorphism.”

The bracket notations that I’ll use below come from Meijer, Fokkinga and Patterson’s excellent paper Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire. If you enjoy the article, I’d suggest giving it a look!

I’m writing this because I recall encountering these names when learning Haskell early on and being very confused, particularly by the latter two types. Folds (and to a lesser extent, unfolds) are commonplace in Haskell. Hylo- and metamorphisms are also pretty common, but they’re not as easy to spot. From Wikipedia:

In computer science, and in particular functional programming, a hylomorphism is a recursive function, corresponding to the composition of an anamorphism (which first builds a set of results; also known as ‘unfolding’) and a catamorphism (which then folds these results into a final return value)

The canonical example of a hylomorphism is the factorial function, which (usually) implicitly composes functions. The goal of this post is to lay out an explicit example of a hylo- (and meta-) morphism in a natural way. We’ll start with a couple of functions:

insert :: Ord a => a -> BST a -> BST a
insert x Empty = Tree Empty x Empty
insert x (Tree left a right)
  | x < a     = Tree (treeInsert x left) a right
  | otherwise = Tree left a (treeInsert x right)

fromList :: Ord a => [a] -> BST a
fromList xs = foldr treeInsert Empty xs

We have an insertion function and our first example of a catamorphism, fromList! We’re folding all values from a list into a new structure (a BST) and destroying the list in the process. This function can be written in so called “banana brackets,” like so: \(fromList = (\!\left|treeInsert\right|\!)\).

IfromList can also be considered an anamorphism. Catamorphisms destroy structures to build final values, whereas anamorphisms take an initial seed value and build a new structure from it. In fromList, xs can be considered a “seed” value to build a BST a from, making fromList a perfectly valid anamorphism as well. As such, fromList can also be written in “lens brackets”: \(fromList = [\!(treeInsert)\!]\).

We can also define a new pair of cata/anamorphisms by folding the tree into a list:

foldTree :: (a -> b -> b) -> b -> BST a -> b
foldTree f b Empty = b
foldTree f b (Tree left a right) = foldTree f ( f a (foldTree f b right) ) left

toList :: BST a -> [a]
toList t = foldTree (:) [] t

foldTree is analogous to foldr (and would be a fine definition for foldr in a Foldable instance), and toList destructs (folds) a BST a into an [a]. Thinking this way, toList again defines a catamorphism, this time from BST a -> [a], denoted \(toList = (\!\left| : \right|\!)\). But we can also think of toList as unfolding a BST a into an [a], so we can analogously define an anamorphism \(toList = [\!( : )\!]\).

There’s something interesting about toList: foldTree traverses a BST in order, so it actually produces a sorted list (given that elements are inserted rather than randomly placed!). Now we have a way to construct a binary search tree from a list of elements, and destruct a binary search tree into a sorted list of elements. This gives rise to a simple definion of a sorting algorithm, namely:

treesort :: Ord a => [a] -> [a]
treesort = toList . fromList

Because toList and fromList are both cata- and anamorphims, treesort actually defines a hylomorphism and a metamorphism.

As we noted before, a hylomorphism is defined as the composition of an anamorphism (unfold) with a catamorphism (fold). If we think of fromList as an anamorphism and toList as a catamorphism, we have constructed a hylomorphism directly. Namely, the function \(treesort = [\![([], (:)),(insert, null)]\!]\) (the brackets here are commonly called “envelopes”). null isn’t explicit in the definition of treesort (instead, it’s implicit in foldr), but it describes a terminating condition for fromList. Similarly, [] is just the container to fold values into.

We can once again think of this function in the opposite manner by thinking of fromList as a catamorphism and toList as an anamorphism, giving rise to a metamorphism, defined by composition in the opposite direction. Metamorphisms (as far as I know) have no bracket notation in literature, but I want to mention that we do have a name for such things. My guess is that any metamorphism can actually be thought of as a hylomorphism, since the objects being operated on must be both foldable and unfoldable, but I don’t know for sure.

Finally, note that we can also create another function:

what :: Ord a => BST a -> BST a
what = fromList . toList

which is also a hylo- and metamorphism. However, this isn’t very useful (in fact, one might consider it counterproductive), but I’ll leave it as an exercise to the reader to figure out why.

Thanks for reading!