[Haskell-cafe] Timing pure functions?

[Magnus Therning]

On Wed, May 27, 2009 at 9:59 AM, Andrew Butterfield
<Andrew.Butterfield at cs.tcd.ie> wrote:
> Magnus Therning wrote:
>>
>>  timeIt times ioa = let
>>          timeOnce = do
>>              t1 <- getCPUTime
>>              a <- ioa
>>              t2 <- getCPUTime
>>              let t = fromIntegral (t2-t1) * 1e-12
>>              return t
>>          in sequence $ take times $ repeat timeOnce
>>
>>  main = do
>>      fh <- openBinaryFile "/dev/urandom" ReadMode
>>      d <- liftM BS.unpack $ BS.hGet fh 100000
>>      t <- timeIt 10 $ return $! B64.encode d
>>      print t
>>
>>
>> I suspect that it all comes from `B64.encode d` being pure, hence the
>> encoding happens only once.  Now I _really_ want the encoding to
>> happen 10 times, is there some easy way to achieve this?
>>
>>
>
> A quick answer - not a lot of thought - pass function *and* argument
> separately into timeIt ?
>
>  timeIt times ioaf ioaarg
>     ....   a <- ioaf ioaarg
>
> As it stands you pass the thunk (B64.encode d) in so it only gets evaluated
> once
> If you pass the function and argument in then a new thunk is built each time
> around
> (unless the optimiser nabbles it.......)

Hmm, my naive implementation of that didn't improve the situation, `t
<- timeIt 10 (\ x -> return $! B64.encode x) d` still results in only
one measurement /= 0.

Of course that also makes `timeIt` less general.

/M

-- 
Magnus Therning                        (OpenPGP: 0xAB4DFBA4)
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