It is definitely the strictness analyzer biting you here. In ghci, the behavior of these two programs is identical (stack overflow). As kalman said, if you replate sum with foldl' (+) 0 in each of these programs, the behavior is still identical (correct).
OK, I could replicate that result. Awesome, thanks a lot!
As a side note, one of the monad laws is this:
return x >>= f = f x
So your two programs are semantically equivalent (there's nothing about IO that forces the evaluation of the list).
OK, thanks, this is an important point. So maybe I should have done this? main = print $ foldl1' (+) $! take 1000000 randFloats My intuition tells me that the $! (and `seq`) just reduces one level (to WHNF?). If so, is there a way to force complete evaluation (so that nothing is reducible anymore)? Thanks, Milos