[Haskell-cafe] Fusion for fun and profi (Was: newbie optimization question)

[Stefan O'Rear]

On Sun, Oct 28, 2007 at 01:25:19PM -0700, Don Stewart wrote:
> Finally, we can manually translate the C code into a confusing set of nested
> loops with interleaved IO,
> 
>     main = loop 1 
>       where
>         loop !i | i > 10000 = return ()
>                 | otherwise = if i == go i 0 1 then print i >> loop (i+1) 
>                                                else loop (i+1)
> 
>         go !i !s !j | j <= i-1 = if i `rem` j == 0 then go i (s+j) (j+1)
>                                                 else go i s (j+1)
>                  | otherwise = s
> 
> And we get *no speed benefit* at all!
> 
>     time ./A-loop  1.24s user 0.00s system 98% cpu 1.256 total
> 
> So the lesson is: write in a high level style, and the compiler can do the work
> for you. Or, GHC is pretty smart on high level code.

IO blocks unboxing in GHC.  How fast is your mock-C code refactored to
do IO outside of the loops only?

Stefan
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