This code produces a stack overflow in ghci when I call `makeSpiral' with large values, e.g. big enough to produce a 1001x1001 spiral. (makeSpiral produces a list of lists which form a clockwise 'spiral', it's a puzzle from mathschallenge.net.) I'm sure there is a way to increase the stack space in ghc which I will look into, but is there a way I could avoid the problem in the first place by attacking the problem differently? Does stack space run out because the list is an argument being passed around (1001x1001 versions of it)? If so would the state monad help me? data Dir = R | D | L | U deriving (Show, Eq, Enum) type Spiral = ([[Int]], Int, Dir) -- (rows, current row, next direction) rows :: Spiral -> [[Int]] rows (rs, i, d) = rs currentrow :: Spiral -> Int currentrow (rs, i, d) = i nextdir :: Spiral -> Dir nextdir (rs, i, d) = d getrow :: Int -> [[Int]] -> Maybe [Int] getrow i sp = if i < 0 || i >= length sp then Nothing else Just (sp!!i) ndir :: Dir -> Dir ndir d = if d == U then R else succ d newsp :: Spiral newsp = ([[1]], 0, R) makeSpiral :: Int -> Spiral makeSpiral i = makeSpiral' 2 newsp where makeSpiral' j sp = if j > i then sp else makeSpiral' (j+1) (update j sp) update :: Int -> Spiral -> Spiral update i (sp, cr, d) = (sp', cr', d') where oldrow = if (d == U && cr' == cr && cr == 0) || (d == D && cr' == length sp) then [] else fromJust $ getrow cr' sp cr' | d == L || d == R = cr | d == U = if cr == 0 then 0 else cr-1 | otherwise = cr+1 cr'' = if d == U && cr == 0 then -1 else cr' sp' = insertrow cr'' newrow sp newrow = case d of R -> oldrow++[i] D -> oldrow++[i] L -> i:oldrow U -> i:oldrow d' | d == R || d == L = if length oldrow == maximum (map length sp) then ndir d else d | d == U = if cr'' == -1 then ndir d else d | otherwise = if cr' == length sp then ndir d else d insertrow :: Int -> [Int] -> [[Int]] -> [[Int]] insertrow i r rs = if i == -1 then r:rs else front++[r]++back where (front, rest) = splitAt i rs back = if null rest then [] else tail rest printSpiral :: Spiral -> IO () printSpiral (sp, i, d) = putStrLn (concat $ intersperse "\n" (map show sp)) sumdiags :: Spiral -> Int sumdiags (sp, i, d) = (sumdiags' 0 0 (+1)) + (sumdiags' 0 end (subtract 1)) - centre where row1 = sp!!0 end = length row1 - 1 halfx = (length row1 `div` 2) halfy = (length sp `div` 2) centre = (sp!!halfy)!!halfx sumdiags' row col f = if row == length sp then 0 else (sp!!row)!!col + sumdiags' (row+1) (f col) f -- View this message in context: http://www.nabble.com/ghci-stack-overflow-tf2666036.html#a7435185 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.