Game of life produced on the last coding session of Kyiv Global Day of Coderetreat 2014

game-of-life.hs

import Data.List

-- Transform cells into next iteration
nextTurn :: (Integral a) => [(a,a)] -> [(a,a)]
nextTurn space = [(x, y) | (x,y,_) <- [head xs | xs <- cells space, length xs == 3 || (length xs == 4 &&  True `elem` ([x | (_,_,x) <- xs]))]]

-- Cells grouped by their coordinates
cells :: (Integral a) => [(a,a)] -> [[(a,a,Bool)]]
cells space = groupBy (\(x1,y1,_) (x2,y2,_) -> x1 == x2 && y1 == y2) (sort (neighbourhood space))

-- List of alive cells and their neighbours
neighbourhood :: (Integral a) => [(a,a)] -> [(a,a,Bool)]
neighbourhood space = [ (x, y, True) | (x, y) <- space ] ++ neighbours space

-- List of neighbour cells marked as dead
neighbours :: (Integral a) => [(a,a)] -> [(a,a,Bool)]
neighbours space = [ (x+dx, y+dy, False) | (x,y) <- space, dx <- [-1..1], dy <- [-1..1], dx /= 0 || dy /= 0 ]

Why length xs == 3 || (length xs == 4 && Trueelem([x | (_,_,x) <- xs]))?
If I understand correctly it should contains: length xs == 2 && Trueelem`

Actually, the case when we have alive cell with two neighbours falls into the first part of the quoted condition (1A + 2D), as well as newborn cell. Second part threat specially alive cell with three neighbours (1A + 3D).

I've tested it one more time on Toad oscillator, it looks like the thing is working.

Where are the unit tests?

The solution was developed interactively (and iteratively) using GHCi.

Honestly, I have very little experience with Haskell and don't know how to write unit tests yet.