[Haskell-cafe] Optimization flag changing result of code execution

I was trying to solve a computational problem form James P Sethna's book Statistical Mechanics: Entropy, Order Parameters, and Complexity[1]. The problem is on page 19 of the pdf linked and is titled Six degrees of separation. For it I came up with this code: http://hpaste.org/84114 It runs fine when compiled with -O0 and consistently yields an answer around 10, but with -O1 and -O2 it consistently gives an answer around 25. Can somebody explain what is happening here? [1] http://pages.physics.cornell.edu/~sethna/StatMech/EntropyOrderParametersComp... Azeem