I'm trying to solve this problem: http://projecteuler.net/index.php?section=problems&id=44, and my program runs out of memory. I was wondering where I have a space leak, I thought that this program doesn't accumulate any data structures (i.e. should use constant memory): CODE BEGINS -- returns list of pairs (i, j) of indices, for which differences (gen j - gen i) -- increase, provided that gen has the following property: -- gen (i+1) - gen i > gen i - gen (i - 1) incDiffPairs gen = iterate next (0, 1) where next (i, j) = if (i == 0) || gen (j+1) - gen j < gen j - gen (i - 1) then (j, j + 1) else (i - 1, j) -- returns ith pentagonal number pentagonal i = let n = i + 1 in n * (3 * n - 1) `div` 2 -- tests if a number is pentagonal isPentagonal x = let d = 24 * x + 1 in isSquare d && (1 + sqrti d) `mod` 6 == 0 result44 = head [(pentagonal j - pentagonal i, (i, j)) | (i, j) <- incDiffPairs pentagonal, isPentagonal (pentagonal j - pentagonal i), isPentagonal (pentagonal j + pentagonal i)] CODE ENDS