On Thu, Apr 10, 2014 at 9:48 AM, Stijn Muylle wrote:

All,

I ran into a small problem using guards, and I don't really understand what I'm doing wrong.

I have this code:

---------------------------------- import Data.List.Split type Cell = Bool data Board = Board [[Cell]] deriving Show

trueList = True : trueList

createBoard :: Int -> Int -> Board createBoard x y = Board (chunksOf x (take (x * y) trueList))

getCellAt :: Int -> Int -> Board -> Cell getCellAt x y (Board b) | x >= (length b) = False | y >= (length (b !! 0)) = False | otherwise = (b !! x) !! y

-----------------------------------

When I try to execute this:

getCellAt (-1) 0 $ createBoard 3 3

I get a negative index exception. However, as -1 is smaller than 3 (length of the board), I would expect False to be returned.

Am I overlooking something?

I might be looking at it wrong, but won't your code go through 1. -1(x) >= 3(length b) ---> not true, so check next guard 2. 0(y) >= 3(length $ b!! 0) ---> not true, so check next guard 3. True(otherwise) ---> true /M

Found the problem: I had a bug in my implementation (see Marcus' comment). The solution provided by Dan (using a 2d array) seems better, but right now I'm just focussing on making it work ;) On Thu, Apr 10, 2014 at 9:48 AM, Stijn Muylle wrote:

All,

I ran into a small problem using guards, and I don't really understand what I'm doing wrong.

I have this code:

---------------------------------- import Data.List.Split type Cell = Bool data Board = Board [[Cell]] deriving Show

trueList = True : trueList

createBoard :: Int -> Int -> Board createBoard x y = Board (chunksOf x (take (x * y) trueList))

getCellAt :: Int -> Int -> Board -> Cell getCellAt x y (Board b) | x >= (length b) = False | y >= (length (b !! 0)) = False | otherwise = (b !! x) !! y

-----------------------------------

When I try to execute this:

getCellAt (-1) 0 $ createBoard 3 3

I get a negative index exception. However, as -1 is smaller than 3 (length of the board), I would expect False to be returned.

Am I overlooking something?

Thanks! Stijn