Re: Re[4]: [Haskell-cafe] In-place modification

15 Jul 2007


      On 15/07/07, Donald Bruce Stewart  wrote:
...
hughperkins:
...
Hey, I just realized I can shave off another 30% in C# ;-)
   So now the timings become:
Ok. So do the same thing to the Haskell program. The compilers should
produce pretty much identical assembly.
{-# OPTIONS -O2 -optc-O -fbang-patterns #-}
import Control.Monad.ST
    import Data.Array.ST
    import Data.Array.Base
    import System
    import Control.Monad
    import Data.Bits
main = print (pureSieve 10000000)
pureSieve :: Int -> Int
    pureSieve n = runST( sieve n )
sieve n = do
        a <- newArray (0,n-1) True :: ST s (STUArray s Int Bool)
        let cutoff = truncate (sqrt (fromIntegral n)) + 1
        go a n cutoff 2 0
go !a !m cutoff !n !c
      | n == m    = return c
      | otherwise = do
              e <- unsafeRead a n
              if e then
                if n < cutoff
                    then let loop !j
                              | j < m     = do
                                  x <- unsafeRead a j
                                  when x $ unsafeWrite a j False
Surely you can remove the read here, and just always do the write?

-- 
Sebastian Sylvan
+44(0)7857-300802
UIN: 44640862