Mikhail,
main = do print $ foldl' (+) 0 $ take 100000000 [1.0,1.0..]
works 10 times faster than your C version. You just need to adapt to the radically different style of programming.
My wasn't intended to represent the problem that I'm trying to solve, but the approach I want to take. The problems that I do want to solve don't lend themselves to this kind of approach. My real situation is that I want to write code that has both a high-level component and a low-level number-crunching component that works on large dense and sparse arrays. Idiomatic Haskell is great for the high-level component. But the question is whether or not its worth trying to write the low-level code in Haskell or whether I should just write that code in C and use the FFI. There are still advantages to using Haskell even if the code is highly unidiomatic: you have the freedom of throwing in higher order functions from time to time in the low-level code, and writing mixed-language code is a pain. -- Dan