[Haskell-cafe] Question regarding deepseq (Control.DeepSeq)

[Frank Moore]

Thanks for the reply.

My main is now:

main = do
  let as = [1..2e7] :: [Double]
  start <- getCurrentTime
  let meanAs = mean as
  let meanOver2 = meanAs `deepseq` meanAs / 2
  end <- getCurrentTime
  putStrLn (show (end `diffUTCTime` start))
  start <- getCurrentTime
  putStrLn (show meanOver2)
  end <- getCurrentTime
  putStrLn (show (end `diffUTCTime` start))

The time is still spent on the show meanOver2 command :(

Frank

On Thu, Jun 24, 2010 at 9:11 PM, Ivan Miljenovic
<ivan.miljenovic at gmail.com>wrote:

> On 25 June 2010 10:57, Frank Moore <fmoore at gmail.com> wrote:
> > Hello Haskellers,
> >
> > I am new to programming in Haskell and I am having trouble understanding
> > exactly when statements become evaluated.  My goal is to try and measure
> how
> > long a computation takes without having to use a show function.  The code
> I
> > am trying to use is below (taken in part from RWH chapter 25)
> >
> > ----------------------------------
> > import Data.List (foldl')
> > import Data.Time.Clock (diffUTCTime, getCurrentTime)
> > import Control.DeepSeq (deepseq)
> >
> > mean :: [Double] -> Double
> > mean xs = s / fromIntegral n where
> >     (n,s) = foldl' k (0,0) xs
> >     k (n,s) x = n `seq` s `seq` (n+1,s+x)
> >
> > main = do
> >   let as = [1..1e7] :: [Double]
> >   start <- getCurrentTime
> >   let meanOver2 = deepseq (mean as) `seq` (mean as) / fromIntegral 2
> >   end <- getCurrentTime
> >   putStrLn (show (end `diffUTCTime` start))
> >   putStrLn (show meanOver2)
> > -------------------------------------
> >
> > My understanding of deepseq was that it evaluates (mean as) completely
> > before continuing, and then the show would not take any time, but instead
> > all the time is spent in the show meanOver2 function.  I feel like I am
> > missing something fundamental here.  Any suggestions?  Thanks for your
> help.
>
> It does... but because you don't save the result it doesn't keep the
> result (you're wanting Common Sub-expression Elimination, which GHC
> doesn't do).
>
> Try this:
>
> let meanAs = mean as
>    meanOver2 = meanAs `deepSeq` meanAs / 2
>
> Note that your usage of "fromIntegral" isn't required, as that is
> automatically done for any integral literal (but if you had 2 in an
> Int variable then you would need fromIntegral).
>
>
> --
> Ivan Lazar Miljenovic
> Ivan.Miljenovic at gmail.com
> IvanMiljenovic.wordpress.com
>



-- 
Dr. W. Frank Moore
H. C. Wang Assistant Professor
Department of Mathematics, Cornell University
310 Malott Hall, Ithaca NY 14853-4201, USA

Office: Malott 587, Phone: +1 607 255 4030
Email: frankmoore at math.cornell.edu
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