If one thinks about Haskell data structures as of ordinary data structures, fusion seems a great deal -- instead of producing intermediate lists and possibly running out of memory, we just run a loop and use constant amount of space. But Haskell data structures are quite different -- they are produced as demanded. Consider the example from the Stream Fusion paper[1]: f :: Int → Int f n = sum [ k ∗ m | k ← [1..n], m ← [1..k ] ] Assuming the sum is a strict left fold, it consumes elements of lists one-by-one and runs in constant space. The list part can be transformed to foldr (++) [] $ map (\k -> map (\m -> k*m) [1..k]) [1..n] which is capable of producing elements one-by-one. So the whole thing probably should run in constant space as well. Of course I don't claim that fusion is useless -- just trying to understand the problem it solves. Are we saving a few closures and cons cells here? [1] Stream Fusion. From Lists to Streams to Nothing at All. Duncan Coutts, Roman Leshchinskiy, Don Stewart. -- Roman I. Cheplyaka :: http://ro-che.info/ Don't worry what people think, they don't do it very often.