Daniel,

This means *when meanOver2 is evaluated*, then evaluate (mean as). Binding it in a let is lazy, so it won't be evaluated until it's needed (for printing in this case). Also note that (mean as) is a Double, so deepseq is just seq in this case (but I suppose this is just a boiled down example and you also want to time computations with results where deepseq does strictly more than seq).

Thanks for the help, I think I understand now.  This is indeed a boiled down example.  I have some data that I am trying to compute in parallel, and I want to control how much evaluation to force.  To do this, I was trying to use NFData and deepseq, since I thought bang patterns were a little clunky (but maybe they are less clunky than forcing evaluations with seq and deepseqs).  Are there best practices for keeping track of which variables are bound where, when writing a complex program?

There are two standard ways to achieve what you want, 1.   let meanOver2 = ...    end <- meanOver2 `deepseq` getCurrentTime

2. put {-# LANGUAGE BangPatterns #-} at the top of the file and write    let !meanOver2 = ...    end <- getCurrentTime

The bang on meanOver2 means "evaluate this expression now (to weak head normal form, i.e. to the outermost constructor)".

Sometimes for me, whnf is not quite enough.  For example, if I have a newtype P (with type constructor P) that is a for Map M Int.  Is NFData and deepseq the way to do this, or should I be trying to implement my own Strategy along the lines of rwhnf in Control.Parallel.Strategies?

Another thing, for timing computations, wall-clock time is not appropriate, better use

System.CPUTime.getCPUTime

to get only the CPU-time the process took, and not also what your browser or whatever used in the meantime.

Thanks for the tip. Frank

On Thu, Jun 24, 2010 at 9:38 PM, Daniel Fischer wrote:

...

On Friday 25 June 2010 02:57:31, Frank Moore 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

This means *when meanOver2 is evaluated*, then evaluate (mean as). Binding it in a let is lazy, so it won't be evaluated until it's needed (for printing in this case). Also note that (mean as) is a Double, so deepseq is just seq in this case (but I suppose this is just a boiled down example and you also want to time computations with results where deepseq does strictly more than seq).

There are two standard ways to achieve what you want,

1.    let meanOver2 = ...    end <- meanOver2 `deepseq` getCurrentTime

2. put {-# LANGUAGE BangPatterns #-} at the top of the file and write

   let !meanOver2 = ...    end <- getCurrentTime

The bang on meanOver2 means "evaluate this expression now (to weak head normal form, i.e. to the outermost constructor)".

...

  end <- getCurrentTime   putStrLn (show (end `diffUTCTime` start))   putStrLn (show meanOver2) -------------------------------------

Another thing, for timing computations, wall-clock time is not appropriate, better use

System.CPUTime.getCPUTime

to get only the CPU-time the process took, and not also what your browser or whatever used in the meantime.

...

My understanding of deepseq was that it evaluates (mean as) completely before continuing, and then the show would not take any time, but

No, it evaluates (mean as) completely *when meanOver2 is demanded*, not before.

...

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.

Frank

-- 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@math.cornell.edu

-- 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@math.cornell.edu