# Brief introduction to Conway Game of Life with Comonads

In this article we take a look at Conway’s Game of Life as a good example for comonad usage. We will go step-by-step through basic concepts finally presenting an example of Conway’s Game of Life implementation.

## Background

Functor is a polymorphic type that is able to wrap values in it. These values can be mapped over using some function. This function is often called `map`

or `fmap`

, depending on language’s convention. In Haskell, this kind of type belongs to `Functor`

type class.

```
class Functor f where
fmap :: (a -> b) -> f a -> f b
```

The simplest example you have probably dealt with is a list. List of values can be simply mapped over by iterating over them, applying function to it and packing them back to the list (to immutable copy or modifying existing list).

Also well known case is `Maybe`

functor. It allows to do null-safe computations without tedious checking of null references with control flow statements. Also there is richer functor called `Either`

that allows to return a failure with error details attached (`Left`

) or can wrap a value coming from successful computation (`Right`

).

We can say that functor gives value (or values) a *context* that attaches specific behavior to it.

### Comonads

Comonads, as name suggests, is a monad, but in an opposite category. By definition, if we have function `a -> b`

, then in opposite category it would be `b -> a`

. So, let’s rephrase `Monad`

type class into `Comonad`

type class using this play:

```
class Functor m => Monad m where
return :: a -> m a
join :: m (m a) -> m a
bind :: (a -> m b) -> m a -> m b
class Functor w => Comonad w where
coreturn :: w a -> a
cojoin :: w a -> w (w a)
cobind :: (w a -> b) -> w a -> w b
```

Comonads can be thought as a universe of values that has one distinguished point. To get that point we can call `coreturn`

.

`cobind`

corresponds to `bind`

function and it works like this. `cobind`

takes the universe and for each point creates an instance of comonad that looks at this point. Then `cobind`

calls function of type `w a -> b`

on these copies, resulting a new universe that is made of `b`

-s. `cojoin`

does exactly same thing, but only provides these copies of universes.

You can see that you need implement `coreturn`

and either `cobind`

or `cojoin`

to get the instance of `Comonad`

.

For the rest of article we will call `coreturn`

as `extract`

, `cobind`

as `extend`

and `cojoin`

as `duplicate`

.

### `Store`

comonad

So let consider a pair of values: an index of type `s`

and function `s -> a`

that accesses early mentioned universe of values of type `a`

. This is `Store`

comonad.

```
data Store s a = Store s (s -> a)
```

Shall we implement `Comonad`

type class? Let’s try implement `extract`

.

```
extract :: Store s a -> a
extract (Store idx get) = get idx
duplicate :: (Store s a -> b) -> Store s a -> Store s b
duplicate f (Store idx get) = Store idx (\i -> f (Store i get))
```

You can compare this implementation with the explanation of `extend`

above.

## Game of Life v. `Store`

Game of Life, first of all, is a cellular automata. It has an infinite discrete plane filled with two kind of cells — dead or alive. Each cell in each step is transformed by provided rules, and these rules are:

- if current cell is dead and has three alive neighbours, then cell becomes alive;
- if current cell is alive and has two or three alive neighbours, then cell still is alive;
- otherwise cell becomes dead.

We can start implementing this automaton by describing the board.

```
type Coord = (Int, Int)
type CellPlane a = Store Coord a
data Conway = Dead | Alive deriving (Eq)
```

### Game Of Life is an `experiment`

…

We need to obtain the state of neighbouring cells around the focused cell. We can implement function called `experiment`

in this way:

```
experiment :: Functor f => (s -> f s) -> Store s a -> f a
experiment f (Store idx get) = fmap get (f idx)
```

This function passes the current index into the function `s -> f s`

resulting with `f s`

. Since `f`

is a functor, we can map `f s`

with `get`

.

If we’ll consider a list as our functor `f`

, this gives as a way to get neighbourhood.

```
neighboursOf :: Coord -> [Coord]
neighboursOf (x, y) = [ (x + d, y + d') | d <- [-1..1], d' <- [-1..1], (d, d') /= (0, 0) ]
neighbouringCells :: CellPlane Conway -> [Conway]
neighbouringCells = experiment neighboursOf
```

### Game Of Life *rules*

So now we can easily write the rules of Game Of Life.

```
golStep :: CellPlane Conway -> Conway
golStep p = case extract p of
Dead | noOfNeighbours == 3 -> Alive
Alive | noOfNeighbours `elem` [2, 3] -> Alive
_ -> Dead
where
-- @noOfNeighbours@ = number of neighbouring cells that are alive
noOfNeighbours = length (filter (== Alive) (neighbouringCells p))
```

Now we need to embed an initial state (given by `Map Coord Conway`

) into the comonad…

```
toCellPlane :: Map Coord Conway -> CellPlane Conway
toCellPlane cs = State (0, 0) access
where
-- If we are out of bounds we assume that cell is dead.
access (x, y) = fromMaybe Dead (Map.lookup (x, y) cs)
```

…as well as we want to extract a finite view of the cell plane.

```
data Rect = Rect { x :: Int
, y :: Int
, w :: Int
, h :: Int
}
fromCellPlane :: CellPlane Conway -> Rect -> Map Coord Conway
fromCellPlane p (Rect x y w h) = Map.fromList (coords `zip` experiment (const coords) p)
where
coords = [(i, j) | i <- [x .. x + w - 1], j <- [y .. y + h - 1]]
```

Finally we can create an inifite list of evolutions using `iterate`

function, that repeatedly calls provided function to it (in our case it’s `extend golStep`

):

```
evolutions :: CellPlane Conway -> [CellPlane Conway]
evolutions = iterate (extend golStep)
```

With these three functions — `from/toCellPlane`

and `evolutions`

— we can implement a program that runs Game of Life.

## Summary

Undoubtly, this is an interesting way to describe various cellular automata, using ideas that comes from category theory. Also comonads provides a way to generalise cellular automata to other spaces than just infinite plane.

Only disadvantage of this solution is an exponential growth of running time, because at each `extend`

ing the state of the plane is computed from scratch and memoising these computations could be useful. This can be easily achieved in Haskell using trie-based pure memoisers or in Scala using a mutable `HashMap`

to memoise partial results.