Help: Stack-overflow and tail-recursive functions

[Koji Nakahara]

On Wed, 18 Jun 2003 17:36:28 -0700
"Hal Daume" <t-hald@microsoft.com> wrote:

> Note that there is essentially no difference between f1 and f2.  When
> you $! in f2, all it does is ensure that the argument isn't undefined.
> It doesn't evaluate any of the list.  Try $!! from the DeepSeq module or
> write your own list-forcing function.

Thank you very much.  I understand.

However my original program still (or maybe from the beginning) stack-overflows
at another point, in the middle of the evaluation of "forpaintbdry".

Please give me some advice.
-----------
-- snippet of the program for painting a random matrix from its boundary. 
module Main  where
import System
import Random
import Array
import Ix
import List

main = putStrLn $ show $ forpaintbdry $ rmat 200

forpaintbdry m = [(pos, Live) | pos <- (uncurry bdryidxlist) $ bounds m , isUnknown $ m ! pos ]

bdryidxlist :: (Int, Int) -> (Int, Int) -> [(Int, Int)]
bdryidxlist (a1, a2) (b1, b2) = nub $ [(ab, j) | ab <- [a1, b1], j <- [a2..b2]] ++ 
				[(i, ab) | ab <- [a2, b2], i <- [a1..b1]]

rmat n =    listArray ((1,1),(n,n)) $ map ct (randoms (mkStdGen 1) ::[Bool]) 
	    where   ct True = Unknown
		    ct False = Dead

data CellColor = Live | Unknown | Dead

isUnknown Unknown = True
isUnknown _ = False

instance Show CellColor where
    show Live = "Live"
    show Unknown = "Unknown"
    show Dead = "Dead"