[Haskell-cafe] In-place modification

[Sebastian Sylvan]

On 15/07/07, Donald Bruce Stewart <dons at cse.unsw.edu.au> 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