[Haskell-cafe] In-place modification

[Hugh Perkins]

On 7/15/07, Sebastian Sylvan <sebastian.sylvan at gmail.com> wrote:
>
> > "unsafe"' here just means direct array indexing. Same as the other
> > languages. Haskell's 'unsafe' is a little more paranoid that other
> > languages.


Yes, I was kindof hoping it was something like that.  Cool :-)


> > Since the goal is to flip bits very quickly in the cache, you could
> > localise this to the ST monad then, as its perfectly pure on the
> > outside.
>

Ok, awesome!

J:\dev\haskell>ghc -fglasgow-exts -O2 -o PrimeDonald2.exe PrimeDonald2.hs

J:\dev\haskell>primedonald2
number of primes: 664579
Elapsed time: 0.7030000000000001

{-# OPTIONS -O2 -fbang-patterns #-}

import Control.Monad.ST
import Data.Array.ST
import Data.Array.Base
import System
import Control.Monad
import Data.Bits
import System.Time
import System.Locale

pureSieve :: Int -> Int
pureSieve n = runST( sieve n )

sieve n = do
       a <- newArray (2,n) True :: ST s (STUArray s Int Bool) -- an array of
Bool
       go a n 2 0

go !a !m !n !c
      | n == m    = return c
      | otherwise = do
              e <- unsafeRead a n
              if e then let loop !j
                              | j < m     = do
                                  x <- unsafeRead a j
                                  when x (unsafeWrite a j False)
                                  loop (j+n)

                              | otherwise = go a m (n+1) (c+1)
                        in loop (n `shiftL` 1)
                   else go a m (n+1) c

calculateNumberOfPrimes :: Int -> Int
calculateNumberOfPrimes = pureSieve

gettime :: IO ClockTime
gettime = getClockTime

main = do starttime <- gettime
          let numberOfPrimes = (calculateNumberOfPrimes 10000000)
          putStrLn( "number of primes: " ++ show( numberOfPrimes ) )
          endtime <- gettime
          let timediff = diffClockTimes endtime starttime
          let secondsfloat = realToFrac( tdSec timediff ) +
realToFrac(tdPicosec timediff) / 1000000000000
          putStrLn( "Elapsed time: " ++ show(secondsfloat) )
          return ()
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20070715/bf539ee1/attachment.htm