[Haskell-cafe] Is fusion overrated?

Ryan Ingram ryani.spam at gmail.com
Wed May 18 08:04:45 CEST 2011

Yes, the goal isn't so much to improve complexity (both are O(1)) but to
reduce the constant factor on that O(1).

In an inner loop like that, allocator/gc calls by far dominate the cost of
the program.  If you can remove them, you've improved the performance of the
program by 10-100x.

In the case where everything is Int, you can even unbox and get entirely in
registers, which gives you comparable performance to a hand-tuned C or
assembly language loop.

  -- ryan

On Tue, May 17, 2011 at 10:55 PM, Roman Cheplyaka <roma at ro-che.info> wrote:

> 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.
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