# What I learned about Codata

Posted on December 9, 2020
Tags: explanation Categories: category theory haskell

## Codata as dual of data

### Data

When we define a new type in Haskell, we use the data keyword:

data Foo
= Foo1 Int String
| Foo2 Bool

This is essentially equivalent as defining two data constructors (they are legit functions):

Foo1 :: Int -> String -> Foo
Foo2 :: Bool -> Foo

This type of construct is called “data” by some people.

### Usage of a data

Here’s an example of an inductively defined data structure.

data Tree = Leaf | Node Tree Int Tree
-- i.e.
Leaf :: Tree
Node :: Tree -> Int -> Tree -> Tree

What does it mean to use such data? It means we need to destruct the data by matching on all its constructors:

height :: Tree -> Int
height Leaf = 0
height (Node left _val right) = 1 + max (height left) (height right)

This types of function is called an “eliminator”.

### Data eliminator and initial algebras

A data is an initial algebra we previously learned about. Take the above Tree as an example, it’s defined as the initial algebra on the following functor.

data TreeF a = Leaf | Node a Int a
type Tree = Fix TreeF

You can review the my previous talk on initial algebra.

Then above defintion of height can be expressed as catamorphism on an algebra of TreeF.

heightAlg :: TreeF Int -> Int
heightAlg Leaf = 0
heightAlg (Node left _val right) = 1 + (max left right)

height :: Fix TreeF -> Int
height = cata heightAlg

### Codata

If we revert the arrows that constructs a data, we get two functions dual to the previous data constructors that have the shapes like this:

Bar1 :: Bar -> (Int, String)
Bar2 :: Bar -> Bool

Intuitively, Bar1 and Bar2, instead of two ways to construct a Bar from their components, they specifies two ways to use the Bar. This is what we called codata.

### Construction of codata

Here’s another example of useful codata:

type Stream = (Int, Stream)

tail :: Stream -> Stream

Here’s an example on how to construct this codata.

startFrom :: Int -> Stream
startFrom n = (n, startFrom (n + 1))

### Terminal coalgebra and codata constructor

Above codata Stream can be expressed as terminal coalgebra (fix-point) on the following

type StreamF a = (Int, a)
type Stream = Fix StreamF

Then the startFrom constructor for Stream will be an anamorphism on some coalgebra:

startFromCoalg :: Int -> StreamF Int
startFromCoalg n = (n, n + 1)

startFrom :: Int -> Fix StreamF
startFrom = ana fromCoalg

## General purpose codata

### Church encoding of data types

One intriguing topic in functional programming is Church encoding. Church encoding shows us ways to encode any data and control constructs as purely lambda functions.

\f x -> x               -- 0
\f x -> f x             -- 1
\f x -> f (f x)         -- 2
\n -> \f x -> f (n f x) -- succ :: Nat -> Nat

\x y -> x               -- true
\x y -> y               -- false
\p -> \a b -> p b a     -- not :: Bool -> Bool
\p a b -> p a b         -- if :: Bool -> a -> a -> a

\x y z -> z x y         -- pair :: a -> b -> pair
\p -> p (\x y -> x)     -- fst :: pair -> a

### General eliminator of Bool

Let’s take a look in an eliminator for Bool.

data Bool = True | False
type BoolC a = (a, a) -> a

elimBool :: Bool -> BoolC a
elimBool True  (a, b) = a
elimBool False (a, b) = b

You may recognize that the BoolC for elimBool is equivalent to the Church encoding for Bool. We will show their equivalence in next section.

You may also recognize that that elimBool is the most general eliminator for type Bool. In other words, every valid eliminator can be derived from this eliminator.

In fact, the general eliminator elimBool is the catamorphism of type Bool.

### Isomorphism between Church encoding and the data

We will demonstrate that Bool and BoolC are indeed isomorphic:

from :: Bool -> BoolC a
from = elimBool

to :: BoolC a -> Bool
to f = f (True, False)

It’s easy to prove that from . to = id and to . from = id so I’ll elaborate. So far we have shown that BoolC is indeed a Church encoding for Bool.

### General eliminator for Tree

Let’s look at a more complex type:

data Tree = Leaf | Node Tree Int Tree

type TreeC a = a -> ((a, Int, a) -> a) -> a
elimTree = Tree -> TreeC a
elimTree Leaf                f g = f
elimTree (Node left n right) f g = g left' n right'
where left'  = elimTree f g left
right' = elimTree f g right

You may recognize that elimTree is the catamorphism for Tree. Also TreeC is a legit Church encoding for Tree.

### Visitor pattern on Tree

Now we learned how to find Church encoding and shown how Church encoding is isomorphic as the represented data type.

We can discover a pattern such that we can extract of all the ways we can eliminate a Tree into a single entity. We will call this entity “TreeVisitor”.

type TreeVisitor a = (a, (a, Int, a) -> a)

visitLeaf :: TreeVisitor a -> a
visitNode :: TreeVisitor a -> (a, Int, a) -> a

As the name suggests, this pattern is just the Visitor pattern in OOP. Here I am using a pair to represent the type for TreeVisitor, but the exact way to implement it doesn’t really matter.

The point of TreeVisitor a being a codata type because the only thing we care about it is to be able to derive the two methods visitLeaf and visitNode.

### Tree as Codata

A Tree can then be defined as the all possible TreeVisitor a -> a instances (i.e. TreeC a), as we already proved by showing the isomorphism between Tree and TreeC.

walk :: Tree -> (forall a. TreeVisitor a -> a)

This representation of Tree is also a codata because the actual underlying data structure of Tree is hidden from the outside, and the TreeVisitors already defined all the ways to access it.