Hackle's blog

between the abstractions we want and the abstractions we get.

One way to get an intuition for Comonad, is to see it as a **context** with a **focus**. Good examples are Conway's game of life, calculating blur effect on a image, or "You Are The Average Of The Five People You Spend The Most Time With", or, what we can start with, "Passing the ball in a circle".

We want to write a program to record how a group of football players training together by passing the ball in a circle. It goes as "A passes the ball to B, B passes the ball to C ... E passes the ball to A, A passes the ball to B ...", you get the idea.

I am sure you can think of a good solution already but let's say the requirement is to use Comonad, so we need to find the **context** and the **focus**.

The example should be straightforward here - **context** is the group of players, A, B, C, D and E, and the **focus** is the **index** to the current player with the ball. With this in mind, we come to this nonchalant attempt in JavaScript, with some explicit naming.

```
const players = [ 'A', 'B', 'C', 'D', 'E' ];
const passes = players.map((currentPlayer, focus, context) => `${currentPlayer} passes the ball to ${context[(focus + 1) % context.length]}`);
console.log(passes.join(', '));
```

This is almost a Comonad! We have both **context** and **focus**. But not quite - a Comonad instance really needs to encapsulate the focus, which brings us to the the `Haskell`

version here using `NonEmpty`

.

```
players = fromList $ cycle [ 'A' .. 'E' ]
makePass :: NonEmpty Char -> String
makePass (a :| b : _) = a : " passes the ball to " ++ [ b ]
passes = extend makePass players
```

Do you see the **focus** of `NonEmpty`

? It's a bit hidden - as the "head" element. A neat trick.

Now let's look at `extend`

which comes with any Comonad instance. Doesn't it look awfully similar to `map`

?

```
extend :: (w a -> b) -> w a -> w b
map :: (a -> b) -> w a -> w b
```

Note how the mapper function `(w a -> b)`

of `extend`

takes the entire `w a`

, or, the **context** as the argument for mapping to `b`

. Without an index to find each `a`

with, we must build a different `w a`

with its own **focus** for `(w a -> b)`

to do the mapping. That's exactly what `duplicate`

does.

```
> import Control.Comonad
> import Data.List.NonEmpty
Control.Comonad Data.List.NonEmpty> duplicate $ 'A' :| ['B'..'E']
('A' :| "BCDE") :| ['B' :| "CDE",'C' :| "DE",'D' :| "E",'E' :| ""]
```

It builds a list of lists, with the "head" moving towards the "tail" for each list. Remember, "head" is the **focus** of the list. This gives rise to `makePass`

, which you would have noticed, is the mapper function `(w a -> b)`

.

Incidentally, we also use `cycle`

to build an infinite list so there is always the next 2 players for passing the ball (from and to). But be warned a `cycle`

d list won't stop printing in GHCI if you try.

It may seem the Conway's game of life is much more complex than "passing the ball", but it's actually not!

Now that we know `extend`

is much like `map`

, while the latter projects each element directly, `extend`

projects the **context** `w a`

. This alone makes `extend`

a good match for Conway's game of life, where we'll remember, each cell dies or lives depending on its **context** (in the form of neighbouring cells).

Also now we know `w a`

encapsulates a **focus**, so each call to `(w a -> b)`

can result in a different `b`

(or the same `b`

if so desired). To build a `w a`

for each element `a`

, we implement either `duplicate`

or `extend`

.

You'll see the intuition of **context** and **focus** are paying off. There is still the elephant in the room - "passing the ball" deals with a list indexed by integers, but Conway's game of life needs a matrix, indexed by coordinates. Hardly a show stopper.

Thus my solution to Conway's game of life which should hopefully look straightforward by now.

```
type Coord = (Int, Int)
data Game a = Game {
coords :: Matrix a
, focus :: Coord
} deriving (Functor, Show)
instance Comonad Game where
extract (Game coords (r, c))= getElem r c coords
duplicate g@(Game coords focus) = Game games focus where
games = mapPos makeOne coords
makeOne p _ = Game coords p
rule :: Game Bool -> Bool
rule g@(Game coords (r, c)) = liveNbrs == 3 || (isAlive && liveNbrs == 2) where
isAlive = extract g
liveNbrs = length $ filter id neighbours
neighbours = isAliveOne <$> neighbourCoords `at` (r, c)
isAliveOne (r, c) = maybe False id $ safeGet r c coords
```

Just to iterate on the above points,

`Game`

encapsulates a matrix- the
**focus**is an index into the matrix `duplicate`

builds a matrix of`Game`

each focusing on its own index`rule`

takes advantage of`Game`

as the context to calculate the total of live neighbouring cells

The discerning reader would have noticed we don't really need the sexiness of Comonad for Conway's game of life either, much like passing the football, we just need a `map`

for matrix, and it's indeed handy. I'll leave it to you to figure out an implementation - it can be (even) less hassle than the Haskell version.

Of course, Comonad as an abstraction can be much more powerful than just encapsulating an index in a list/matrix. There are more interesting instances such as `Store`

or `ZipperList`

as can be found in the "reference" section below.

I first heard the idea of using Comonad for Conway's game of life casually mentioned in this highly-recommended video by the good Bartosz, then I found this implementations by Dan Oneață with `Store`

, more impressively Representable Store by Chris Penner, and
Zippers by Samuel Gélineau.

The `Zippers`

is a mind bender and totally did my head in. The plain `Store`

features conceptual clarity but suffers from performance penalty. It's better with `Representable Store`

which has to represent `Vector`

that is not itself representable (warning: Category Theory mumbo), and I couldn't help wondering if it can be made more intuitive.

After staring at `extract :: w a -> a`

and `duplicate :: w a -> w (w a)`

long enough, I somehow find the intuition resemble that of fractals, for example Koch's snowflake (from Wikipedia). Be warned this might not be right for you.

Also beware, `duplicate`

usually only goes one level down (or else my head explodes) but I found the intuition helpful.

Of course, the risk of being specific with any example for anything Haskell or Category Theory, is no example can fulfill the whole abstraction. So once you get an intuition with **context** and **focus** (as far as they can take us!) but before stretching them to the breaking point, it's best to accept Comonad as it comes - `(w a) -> w (w a)`

, no more, no less.