[Haskell-cafe] Why my program runs out of memory?

[Vladimir Ch.]

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