Comonad, Zipper and Conway's Game of Life (Part 2) - First Solution

gistfile1.hs

import Data.List

-- worlds are represented by lists
type World a = [a]

waveRule :: Char -> Char -> Char -> Char
waveRule _ l r
    | fromL && fromR = 'X' -- wave from both dir
    | fromL          = '>' -- from l only
    | fromR          = '<' -- from r only
    | otherwise      = ' ' -- nothing
    where
        fromL = l `elem` ">*X"
        fromR = r `elem` "<*X"

nextGen :: World Char -> World Char
                -- the resulting world must be of the same length
nextGen world = take (length world)
                -- take 3 elements, Left, Current, Right in order
                -- and apply the rule
              . map (\ (l:c:r:_) -> waveRule c l r)
                -- for each cell there's a correponding list
                -- from which the first 3 elements can be taken
              $ tails world'
    where
        -- pad the world with spaces
        -- so we can be sure that every cell can grab
        -- its neighboring cell states
        world' = " " ++ world ++ " "

main :: IO ()
main = mapM_ putStrLn (take 20 (iterate nextGen startStr))
    where startStr = "*  >  *   *  <  **<"